You'll be facing a horde of Reaper infantry-level enemies and several Brutes. There is also a stretch of time during which your squadmates will need to hold on their own, so choose carefully and consider bringing squadmates with high health and damage output.
The mission begins onboard the shuttle, where you discuss amongst yourselves how a probe you sent has managed to pinpoint Leviathan's location and how you're supposed to reach it. One of your squadmates will express doubts on bringing Leviathan to aid in the war, and you can answer whichever way you want.
While descending on the ocean planet the shuttle will be disabled by a mysterious pulse. Cortez manages to set the shuttle down at one of the floating wrecks littering the ocean for repairs and you will have free rein of exploring the immediate surroundings. There's a bunch of datapads by the makeshift shelters that tell the grim story of the wreck's crew, along with some items worth picking up for credits (2x pure eezo samples, 2x survey data, star charts - every piece for 2250; might not be all), an Assault Rifle Omni-Blade, M-55 Argus, and a Pistol Cranial Trauma System by a seaside cranny near the shelters. You will also see mysterious spherical artifacts inside some of the shacks and containers; they can be shot at.
A short interval after coming down the ledge from the Kodiak one of your squadmates points at something in the sky. A Sovereign-class Reaper is descending; enemy foot soldiers start pouring in. They will initially deploy from the opposite end of the area from the shuttle, but will later drop all along the water's edge so stay alert for flankers. Be especially wary of the two Brutes who arrive toward the end. Deal with them until Cortez manages to complete repairs and starts providing air support.
It turns out Cortez cannot stay airborne for long due to being hit again by the pulse so he lands the shuttle at the other side of the wreck. There is a mining equipment salvage (1500 credits) just past the part where you have to leap across. Deal with the enemies then go over and talk to Cortez for the next part of the mission. If you haven't managed to get all items in the first pass due to having been interrupted by the Reapers' arrival, now is the chance to backtrack and get everything missed. There's an Ultralight Pistol Materials mod in another nook by the ocean, near the tarp nearest Cortez. Don't forget to take note of the M-560 Hydra by the supply stashes behind Cortez, the med kits, and the Kodiak. Don't pick it up yet: save it for later.
You ask Cortez for the Kodiak's status and he replies that it's inoperable. One of your squadmates suggests pickup by the Normandy, and Cortez points out the same thing will happen to the ship. Since it's impossible to go airborne or be rescued until the problem goes away you will resolve on pressing forward: find Leviathan. Cortez suggests using the Triton ADS mech stored inside a section of the wreck. Since the door to it is sealed, you need to get it open first. To that end, several power sockets are scattered throughout the wreck and Cortez suggests using the shuttle's power cells to give the necessary juice for opening the door.
Reaper forces start landing en masse again. You gain a backpack containing the power cells and you have to bring them to the sockets, stay alive while the cells are being loaded, and come back to Cortez for more until the door finally comes open (Unlike characters in multiplayer, Shepard is capable of sprinting and rolling while carrying the pack and will not drop it). One of the sockets is near the bay door itself, and another is at the seaward side of the wreck in front of the door. You face a multifold problem here: the energy in the sockets is slowly dissipating, necessitating the need to finish as quickly as possible, and you will face lots of resistance between grabbing the cells from Cortez to the sockets. A good time to use the Hydra is when you're nearing the 100% mark and there are Brutes in the way - a panic weapon, if you will.
Once you get the door open you must use the Triton to clear out the enemies: Reapers keep falling if you insist on taking them out conventionally. You may think you've cleared out a "wave", but the Reaper overhead will simply drop others to replace what you've killed. The Triton handles like an Atlas, use your favored moves for dispatching enemies with the mech. Hordes of Brutes will spawn to test the Triton's mettle so be ready accordingly. When the enemies are dealt with walk the mech over to Cortez so he can run a systems check on it.
When Cortez proclaims the mech ready to go, you will have a choice between diving immediately or holding off first. Now is the absolute last chance to go through any items that may have been missed if you choose to procrastinate. In the section where the Triton was stored, you can find some salvaged xenotechnology worth 2250 credits. You can always talk to Cortez whenever you are ready.
Partway through the dive you will begin to lose contact with Cortez on the surface - some kind of interference is cutting you off. You will spend the remainder of the mission alone on the bottom of the ocean. Take time to enjoy the surroundings, the Triton will helpfully fire flares in order for you to find your way around. You will keep going down until you come across the probe you sent earlier. Walk the mech over to it, then it shoots another flare in the distance. A rumble will issue from the chasm not far from you, go to it and prepare to meet Leviathan.
The entity you're talking to will detail its reasons for staying clear of the war. It will give you an outline of its race's history and you can ask it questions critical to the story. When that's done the Leviathan declares that you must remain with them while the Reapers harvest the rest. It's your job as Commander Shepard to convince it otherwise, and it will accept due to its own reasons.
You will engage the Triton's emergency boosters for the ascent and on the surface you will see the fruits of your conversation with the Leviathan: a Brute will turn against another and starts messing with other Reaper forces, and while you escape in the repaired shuttle the Reaper shows up trying to block your path when the pulse that disabled the Kodiak earlier hits the Reaper, causing it to crash into the ocean.
Back aboard the shuttle, you wake up, and your squadmates ask you what happened down there. If Ann Bryson survived her ordeal with Leviathan relatively unharmed, you will converse with her over the shuttle's comm.
The hull will hold--that's what they say. Whatever god watches over eezo prospectors came through for us. Despite a hell of a smack in the water landing, the MSV Monarch displaces enough air to stay afloat. We're all thankful we're alive. Captain Pratt even ordered the champagne broken out. Helps perspective, I guess.
The big puzzle? What downed us in the first place. Kunshan says it wasn't lightning; it was some kind of pulse. A directed-energy weapon seems likely, but where would it have come from? There's no radio traffic. The whole bloody ocean is silent.
Tonight the techs are going to try rerouting the power to get a good mass drop before getting the thrusters firing. If all goes well, we'll be airborne in 24 hours.
Datapad log: Shen Kunshan
Attempts to get airborne have failed every time. The damage done by the pulses, if you could call it damage, doesn't make physical sense. The connections aren't fried, but Monarch's thrusters disengage. The distress beacon is likewise dead, but most of our electronics are still functioning. It doesn't seem to be a result of shielding.
Without a distress beacon, we've got no chance. I hear bad things about ration supplies, and I think that's why the captain ordered us to collect rainwater and set up tents on the deck. It keeps us busy. Brooks has been talking about using the Triton's weapons to try blast-fishing. I agree, but I haven't seen anything that looks like a fish.
I wish I were an optimist.
Datapad log: Dr. Chai Lin
The rations will hold us two days more. Brooks sent a drone to the bottom, and it came up with bioluminescent plant life. None of it is edible. I warned the crew not to eat it before my tests were done, but Romero and Molins tried it anyway. They haven't stopped vomiting, and their dehydration wastes fresh water.
We sent Kunshan's team on a raft over to the wrecks. They came back with spheres--some kind of organic-based alien technology--and nothing to eat. I want to believe they'll give us some answers, but if they had helped the other ships figure out what happened, they wouldn't be here, now would they?
It's getting cold.
Datapad log: Captain Abel Pratt
Ramos is our first death. He tried to throw one of the spheres overboard, and something happened. Kunshan and maybe three or four others started beating him like madmen. I wish I was quick enough to save him.
They showed no remorse, not even comprehension. It's clear I am no longer in control.
I held a funeral to keep order--the crew would have been at his body like wild dogs. We compromised: he will be tethered to the water to attract scavenging animals what we will catch for food. If he doesn't get a bite within three of this planet's days, we cut him loose as a water burial.
We've sealed the Tritons off. They're dangerous, especially given the crew's mental state.
My kingdom for a shark.
Datapad log: Encarna Camacho
The captain gets it now. He tried to lock himself in the ship, screaming that he had to fix it.
We were gentle. One person for each limb. We let him rest on the sphere. Sometime in the middle of the night, he came around, and now he agrees. We won't leave. Why would we?
I think this will be my last entry. Everyone understands me. If anyone finds us, they might feel differently, but that can be fixed in time.
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