I believe you can still underground farm in an ocean biome.
I just tested it to see..
It's true you can farm on muddied stone beneath the ocean, that is, 1Z under the lowest level of water.
However, to actually farm IN the ocean, that is, in a structure surrounded by (or on the top of) the ocean water tiles, it doesn't work.
"No seeds available for this location"
1Z level lower than the ocean, plump helmets no problem. In the water or on the water, can't plant, even though you can create the Farm Plot.
I mean... technically the challenge is: Construct fortress under an ocean. If that means beneath the lowest level of water, that is, not surrounded by water? Sure, but that's not really a challenge. That's just: dig down and over. ![Tongue :P](http://97.107.128.126/smf/Smileys/aaron/tongue.gif)
I take it to mean a significant amount of the fort resting on the floor of the ocean, and not under it. I think for the purpose of the challenge, digging down one level for farmland is probably an allowable exception. Either that, or just utilize the bounty of the ocean.
Using diagonal pressure relief you could create a drainable reception tank and try to lure in oceanic critters, including ye-olde giante whalee. Let's see, for 7 dwarves you need roughly 70 units of food per year. A giant sperm whale produces at least 9,000 units of food. Should only need to catch one of those every 130 years or so. Just gotta make sure there's a butcher shop pretty much right next to it.
Although, even a normal sperm whale is enough for 10 years. I think it's very viable to create a whale-based food solution.
If you get really lucky, you could snag a mating pair of seals and create a complete meat industry, too.
Booze would be the big problem. The only way to create booze under the ocean but above the sea floor would be to keep bees. I hate bees.
Then again You could also feasibly bring whatever animals you want with you to generate food and still be within the rules. 1 turkey hen and gobbler would be enough to produce food in perpetuity via eggs and eventually meat.