Hi!
Polyurethane is good..think it is called "Gorilla glue ". But to cover foam floats with balsa then this metod is best .
Pour out some white glue or aliphatic resin on the balsa sheet that you are going to cover the float with...spread it thin ..let it dry. Do the same on the float. Then when everything has dried...just use your iron to secure the balsa onto the foam float.

But a much better way of having a nice surface is to using Epoxy finnishing resin and 25g-50g glass cloth. Much faster and much more easy to do!

Regards!
Jan K
Sweden