Well friend, you've come to the right place. Lorthos is my favorite deck, and I followed an oceanic theme (or a really big fish tank for arguments sake). Anyway, here is the list: http://tappedout.net/mtg-decks/lorthos-true-god-of-the-sea/

There are cards a little off theme (mainly the artifacts, but I always argue they were in the Tolarian Academy when it was submerged in the sea). If you're going an aquarium theme, some of the cards like [[Winding Canyons]] suddenly fall out of theme (unless your tank is actually a netted off portion of ocean or sea).

What you're going to find is that a lot of Fish creature types are really, really bad cards (think [[Ancient Carp]] bad, then a little worse than that). The same is true for a lot of other sea-type stuff. You may end up having to go big here.

Luckily for you, Lorthos is a damn monster, and would naturally surround himself with other giant badass sea creatures (the little ones in the deck are there because they seek protection from the boss man).

My list can play like a control deck, which I think will happen for yours as well, especially if you stay close to theme. A lot of the little sea dudes tap down or bounce or steal peoples things, then the baddies come in and clean up. I sometimes can get a turn 3 Lorthos (and yeah, its pretty disgusting and you win basically as soon as he enters play in that situation, even on a table with 4 other players), but if you stray from rocks, you probably can't do that (luckily, the new [[Sapphire Medallion]] art has a coastline, +1 for theme baby!).

The creature suite in my deck should give you a lot of what you're looking for. Other permanents and spells are on theme, but you'll have to pick them as you will. I wish you good luck in you're voyage to sea!