I would like this a lot, and have tried some model changing, but is has not worked for me. I do not think the Sims 2 would be able to really help. The references for the body for animations are different, unless you feel like making your own animations. The best bet is to make a marine body with various heads. If my maya autodesk ever worked I would work on this, and other projects, avidly.
If anyone has had any success in changing the human model without altering the base file, please let us know. It has been driving me crazy enough to go back to writing java programs because I know they will work. The model command does not work and from what I can tell, everything is based on classes.