Polyurethane is what they use for flotation, sometimes incorrectly called urethane. I've used this product. Good reading anyway. Only other advise is USE GLOVES! It's like getting super glue all over your hands.
http://www.shopmaninc.com/foam.html
But I'm still not sure you can't just a few cans of Great Stuff from Lowes.
Good luck with your project!
Depends entirely if you are insulating your boat or putting flotation in your boat.
Great stuff is insulating foam. Its also not closed cell. Its like a sponge.
Just any ol foam is not flotation foam.
Trust me. Boats are not something new or is building or restoring them.
Every conceivable way to cut a corner has been tried throughout the years and the vast majority of those cutting corners met with failure or disaster.
We see this all the time in stitch and glue boats particularly in the fillets.
People dont want to spend the money for epoxy and filler so they cut a corner with trying to make fillets with construction adhesive.
It dont work. They've just wasted the entire cost of the hull materials over a couple a hundred bucks.
There have been attempts at building boats using insulating foam sheets instead of using the marine foam sheets designed for building boats with.
It dont work. Its been tried.
The list of failures is long and extensive with loss of real property included.
If the goal is to float the boat dont use anything not labeled as such.
Its that simple.
There are other options. Its hard to beat good ol air for flotation. Sealed airtight compartments will float a boat as well or better then foam will.
Although I wouldnt do it because its just not my style, some people use plastic drink bottles with the tops screwed on to partially fill a compartment and then foam in and around them.
I've not done any testing but the weak point would be in the plastic cap. At some point its going to degrade. Quicker then the bottle itself will.
The whole idea of flotation foam is to float some portion of the boat or the entire boat in an upright attitude.
If my life or anyone else's life is dependent on the foam job I'm putting in there isnt but one way to do it.
The right way, with the right materials. If its expensive, well its just expensive. I'll save my pennies even if it means I may not be able to complete the project for another 6 months or year.
The last thing I want say is that the foam may be critical to the hull.
In other words the foam may be relied on structurally as a stiffener or support. You dont want insulating foam or foam sheets for this job.
An example of a boat that does this is the Carolina Skiff.
Many other boats use foam for sole (floor) support as its lighter then framing.
Once you need that foam job thats not there for you, its too late. Sink or swim.
oooh: I am still planning on useing it, but I will be sealing it in plastic sheeting to protect it. 
