make a small,simple 2 minute water car!.
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!.
blow up the balloon,tie it and tape the ends together
that should make a oval shape.
Step 2: Cardboard Time!
Next cut out a suitable shape to fit the middle of the balloon,then tape it
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,yes you are pretty much done exept it wont move.
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!.