Excuse for Sealab 2021 jokes.
First, while you'll indeed suffocate if you don't have a breath mask (or equivalent) and a source of oxygen, you won't get burnt by radiation or crushed by pressure, even in the Trench. Thus, you don't really need a diving suit or helmet to walk on the surface, though you do need one in the Trench if you don't want to suffer horrific burns. In addition, since there's obviously lots of liquid, swimming gear like flippers and inner tubes dramatically increases how fast you'll walk/run in the water. Similar to Space, Changelings don't have to worry about breathing in it, though they'll still be slowed down, while Vampires burn up from being in it, and Blobs can't spread onto it with the Space Bridge upgrade.
Second, you can move perfectly fine on the ocean floor, no jetpack necessary. Farting with the Farty Thrusts buff and throwing an object still launch you back a square, so you can still enjoy your fancy conservation of momentum tricks. Also, most Vehicles have no trouble treading on the seafloor sand.
Finally, temperatures are a bit weird. If you're in the Trench, you'll eventually freeze up, though nowhere near as fast as in Space. On the other hand, if you're on sea floor of the station z-level, you'll actually warm up slightly.
Now that we've cleared some misconceptions about the seafloor environment, let's discuss the flora and fauna that inhabit it. No doubt you've seen some of Oshan's tropical fish species and perhaps a nautilus or two. Not only do they make delicious sushi, but they're also surprisingly similar to you and I. Attack them, and they will run away. Set them on fire, and they will burn. Grab them, and you can suplex them or choke them out. And if you pull them into some place without water, they will eventually asphyxiate (though it might not look like it).
No doubt you've seen the less charismatic, but still very useful coral and algae life as well. You might know that cutting seaweed gives you delectable seaweed sheets, but did you know that cutting coral yields its skeleton? No one's sure why every species gives the exact same branch, but everybody's pretty sure on how lovely it looks. Also, as it turns out, cutting tube sponges gives, well, sponges, just like in Ye Olden Dayes.
Supposedly, there's also a great white space shark out there that eats people, but that's just a myth...right?