I recently received this question from a member so I contacted a former engineer at Triumph Boats to get the answer. I'm posting here in case this helps others. I will also add this to our Triumph Boats FAQ section.
Triumph used two different density, closed cell, polyurethane foams. In the transoms we used 20lb per cubic foot, high density for strength to help support the motor. The rest of the boat is filled with a 2.5 lb per cubic foot foam for flotation. Both of these foams are "two part" that are mixed at the gun and create the foaming reaction and also result in the closed cell foam. I don't know of any single part foams that are closed cell. Great Stuff, sold at home improvement stores is not closed cell and will absorb water. 2 part polyurethane kits can be bought on line but are several hundred dollars per kit.