Couple of things I found out the hard way that I haven't seen mentioned in these pages:
I had to put a new floor in my older 19 ft Glastron runabout some years ago and used pour-in 2 part Urethane foam for the job. Going by recommendations at the time, I just used to-go drink cups for mixing - 1 each 16 oz to measure the parts, (to an inch or so below the rim) and a 32 oz to mix and pour with.
I did the floor on a warm, sunny summer day and had pre-cut 4" holes in the plywood floor about 3 ft apart, between the longitudinals. Jacked the front of the trailer as high in the air as I could get it, so's the foam mix would run downhill quickly. Watching that stuff was amazing....like the return of the Blob. Pour the 2 - 16 oz cups into the 32 oz cup, (just pour and go - you won't have time to scrape the cups clean) stir it quickly - you'll see the color change and you'll also see it very quickly start to rise in the cup. You gotta work fast.
Pour it into the lowest hole, wait a few seconds and you'll see it coming back up like a living thing as it expands. Kinda spooky/comical. I did it this way since I'd been warned that it would put extreme pressure in a closed container - enuf to rip my floor loose. My 32 oz at a time turned out to be right at the limit.....I heard that floor creak a few times but it did hold. I'd kept the hole saw disks and screwed a cross piece to each ahead of time. As the foam reached each hole, I jammed a disk into that hole and screwed it to the deck to keep the foam contained. "It happens fast ! ! !"
The difference in that boat was amazing. Previously, it was very noisy going thru chop, with the stringers slapping the underside of the floor. Now, it was rock solid and rigid and very quiet. I loved it.
As a follow-up, several years later I built a Sam Devlin design duck boat (original Black Brant - excellent design) for a friend and used the same stuff in the bow floatation compartment and in the rear sponsons. I built that boat in the winter in the northwest and it were cold. When I mixed and poured the 1st batch of foam, it didn't do anything. Oh, it did swell up some, but no more than 2 or 3 times its' original volume....not much good for floatation....and that stuff is expensive.
Finally figured out that it was the temperature.....dohhh. For myself when working with the epoxy on the boat, I'd taken a module out of a Kitchen Aid dishwasher that heated the air for drying dishes. (this was in the early '90s) I put that in a window to pick up clean air and it delivered nice warm air. Attached a 20 ft length of portable oxygen hose to it to carry that warm air to my face mask. Next batch, I put that plastic hose down in the hull with warm air blowing thru it to pre-warm the compartment and set the Urethane containers on the baseboard heaters for an hour or so, till they were nice and warm. Then mixed and poured and they acted as the stuff I used in the runabout did.