I did an ocean theme this year... Here are a few things I did:
-"Fin-Tastic" First Graders bulletin board with student work
-Students had their own fish magnet and chose their fish food every morning (lunch count)
-I bought an octopus at the teacher store and we created our 8 rules for our classroom and I wrote them on the tentacles.
-Students made fish out of foam and cd's and they were hanging from the ceiling...very eye catching (dltk.com)
-I'm sure you know this, but there are gobs and gobs of underwater posters, bulletin boards, etc at the teacher store...I got a lot of my ideas from there and revamped them a little.