Jim Conlin wrote:

Just a minute!
Two-part foam is not necessarily a good solution either.
Some of 'em will soak water, getting heavy and encouraging rot and mildew.
They prevent access for repairs and wiring, etc.
THey're not cheap, either.

Have you considered inflatable items like truck tubes?

Truck inner tubes would make great flotation. They are tough & could be
inserted into any odd shaped space, and removed for repairs/maintenance. but
they're not cheap either.

How about those styrofoam packing peanuts stuffed into a trash bag? I've used
this for flotation and they work well. Price ain't bad either.

Fresh Breezes- Doug King