If the seats are enclosed already (why else would you need inspection holes?) why bother with the expensive foam? You could just fill the enclosed areas with packing peanuts. It would take only a few minutes, there would be no problems from overexpansion of the foam, and probably no cost. Be sure you get the plastic "peanuts" and not the ones made from cornstarch that disintegrate in water.

You can probably find plenty of places that will give you all you want for free.

if the seat is not completely enclosed, you can get a cloth bag (waterproof or mesh -- it hardly matters) and stuff that with the packing peanuts. Lash the foam filled bag under the seats, and you have your flotation.

the same can be done, a bit neater but a bit more expensively, by cutting pieces from a 1 or 2 inch thick foam building insualtion panels. You can get genuine Styrofoam brand (the blue boards) or the generic equivalent, but avoid the "bead" boards. They'll fall apart on ya unless you enclose them.

A tube of a construction cement like "liquid nails" will allow you to stack several layers of theis material.

For your live well or cooler, instead of expensive expanding foam, start with a cheap foam cooler from the local party-goods store. Build a box to tightly surround it out of your 1 inch or 2 inch thick foam panels, bonding the seams and edges with your construction adhesive. The box you build probably won't be watertight, but when you slip in the purchased foam cooler, it will serve as a seamless liner, and with the added inches of foam insulation it will keep things colder for a longer time. A few scraps of wood can be fashioned into a bracket to hold the foam cooler in place, or you can cut away a large section of the deck to install the cooler, and then replace that section as if it were a very large hatch.

Take a look in the archives at the pictures Paul Stomski posted of the triple he constructed. You might get some ideas from his boat. He made his seats from minicell foam and made cockpits that could be covered with large flush hatches.

if the seats are of minicell foam you hardly need to add more foam to them. They already are as bouyant as the stiffer foams.

As for how much flotation you need, well that depends. But that is another story.

PGJ