first thing you need to do is determine EXACTLY what you want to do... seriously, best way to get somewhere is to know where you want to go...
lets look at the swimming first...
it sounds like you want a simple character that swims in water... ok, lets say that that is EXACTLY what you want to do...
next thing to do is determine how you're gonna do it... what, if anything, does 3DRAD give you to help you do this...
well... right off the top of my head i'd the buoyancy object comes to mind... linking it to a rb and an animated mesh should get you going...
then you'll need to sync the swimming actions with movement and animation... this is where the logic comes into play... before you script write in plan language exactly what needs to happen for animastion and movement to work... do one at a time...
when you are satisfied with the logic, then start the scripting... one thing at a time...
you start this and i'll help you... it's not that hard... trust me... just take it one step at a time...
--Mike