if you have Maya Unlimited, there's an Ocean shader you can use, though just saying "Ocean" could mean a lot of things..

do you mean an ocean surface only (for that the above would be nice), or do you mean a complete ocean that you can dive into and make it look like you're underwater...?

the latter can be very complicated, but that's where a tutorial could come in of course.. without more info it's hard to know what you need

ps/ there's another thread somewhere around here that dealt with something like that not too long ago