Small powerboats are required by law to carry foam flotation.  Even boats built from kits, and even boats with sealed flotation compartments!

Read the relevant Coast Guard regulations here.

Read my article about the Peeler's Coast Guard certification here.
 

The use of foam floatation is prominently mentioned. It's just that the approximate amount needed is never specified.   

Ah, just so, Bob.  That IS a good piece of information to know.  The reason we weren't more specific was the variation in the available size and thickness of those foam sheets at the big box stores---it's all over the place.  Perhaps a cubic footage number or something would help when builders get to that moment.

   My hat is off to you, John. I know this is reserecting an oldish post but I clicked on the link to the Coast Guard reg's for boat builders - - - HOLY CRAP!  You need to be a sea lawyer / tax attorney / expert in Chinese algebra to have a chance with that. Being non of the above all I got was - blah blah blah flotation blah blah. Thanks for your efforts on our behalf.