My suggestion for floatation would be to use plastic soda bottles with the caps on tight, or glued on. Put a heavy plastic bag in the compartment that you want to use for floatation, stuff as many soda bottles in the bag as you can, then fill the bag with two-part closed-cell foam (NOT the "Great Stuff" foam that you get at your local Chinamart!).
The reason for the bottles is to take up space and reduce the amount of expensive two-part foam needed. The reason for the heavy plastic bag is 1) another barrier against water infiltration, and 2) contain the bottles and foam. The reason for the foam is to displace air and keep water out, as well as keep bottles in place and keep them from rattling.