Attached Link: http://www.renderosity.com/viewed.ez?galleryid=1205242&Start=1&Artist=anastasis20&ByArtist=Yes

Definately up the contrast and try switching on caustics that should create interesting patterns/highlight to give you more of an underwater feel. The link goes to one of my photos that shows caustics in action. And how about some depth of field, that should highlight the salmon and also help to knock the perma-green trees out of focus.

Full agreement with Mark, "underwater floating stuff" is all important - that's where most of Pixars effort went in creating convincing scenes for Finding Nemo.

"I don't have experience using alpha mesks, except in photoshop." - that could be what you need. Lots of people create grass and hair on a plane with alpha masks. Create your underwater stuff and masks in Photoshop and put it on a plane in Carrara.