I made this out of a cheap boogie board & a couple of old 550 motors. Then added a couple of ducted fans to get boats out of the weeds.
Any slab of foam will do.
[link]http://www.youtube.com/watch?v=rGoRZpx4WyY[/link]
[link]http://www.youtube.com/watch?v=iLpHQi-456c[/link]
All good fun.
With the boat you've got, the idea posted by Misshydro & HendricX is your best option. A tried & tested method all over the world.
Cheers.
Paul.