it needs to be pushed to be moved in water but you could add a engine
or get a balloon strap it on and go!.
Anyway heres what you need!.
Sorry for bad graphics...
Step 1: Balloon Time!.
that should make a oval shape.
Step 2: Cardboard Time!
and yes it does look like a hover craft,and thats good
because you can use it that way.
Step 3: Your Done! Kinda...
Well you can make it move,you can buy one of those cheap
small fans that run on watch batteries you can attach them upside down so it's propellers are have in the water or you can strap a normal balloon to it and go!.