bill wrote:
I'm doing some repairs on my sailboat, an old Oday day sailer, and
when I removed the seat from one side I found a large tube of
styrofoam created with some large version of something like 'great
stuff' and a bunch of small sheets of white styrofoam. Is this
floatation required, beneficial? What type of products are available
to create custom styrofoam under the seat?

Try U S Composites for pourable urethane foam.

You want the 2# stuff which is sold in a two part kit which can be
shipped UPS.

I used U S Composites located in South Florida, am I'm in California.

Good service.

Lew