I have used surfboard foam as boyancy.
I shape a piece of surfboard foam to fit inside the cavity, coat it with resin and fix it in place with a small dob of epoxy. The reason I used surfboard foam is the fact that it is easy to work with and the resin won't disolve it like noodles etc.
I found that if you shape it well, you can coat the inside of the cavity with resin and fit the foam shape into the cavity, let it dry. Then sand the top of the shape flush with the deck, then add the deck.
It strengthened the deck and made for a more rigid boat that floats.

Regards Brettos.