Was a little late as there's a new version now, but no matter.
First off, excellent job so far. I'm really impressed with the skintones and lightuse.
The others have mentioned a few things to make it look a little more 'underwater', I did a quick edit to test some ideas.

The bubbles are the key factor really. I added various fish, some seaweed and those little white things that usually cling to rocky surfaces (Don't know their translated name, sorry.) Actually, I think those only live close to the surface come to think of it. Something similar might work though.
I'm a little confused about how the legs end. I wouldn't have questioned it hadn't it been for the outline.