You'd have to be very sure that you were using a closed cell foam otherwise it would eventually become waterlogged and you'd have a 3 ton RIB whether you wanted it or not
I have no idea if normal builders expanding foam is closed cell or not.
Also read this:
Peter Parry wrote on the Subject of the Expanding Foam
A friend of mine once built a canoe. He spent a long time on it and it was a work of art.
Almost the final phase was to fill both ends with polyurethane expanding foam.
He duly ordered the bits from Mr Glasplies (an excellent purveyor of all things fibreglass) and it arrived in two packs covered with appropriately dire warnings about expansion ratios and some very good notes on how to use it.
Unfortunately he had a degree, worse still two of them. One was in Chemistry, so the instructions got thrown away and the other in something mathematical because in a few minutes he was merrily calculating the volume of his craft to many decimal places and the guidelines got binned as well.
He propped the canoe up on one end, got a huge tin, carefully measured the calculated amounts of glop, mixed them and quickly poured the mixture in the end of the canoe (The two pack expands very rapidly).
I arrived as he was completing this and I looked in to see the end chamber over half full of something Cawdors Witches would have been proud of. Two thing occurred to me, one was the label which said in big letters: "Caution - expansion ration 50:1" (or something similar) and the other that the now empty tins said "approximately enough for 20 small craft"
Any comment was drowned out by a sea of yellow brown foam suddenly pouring out of the middle of the canoe and the end of the canoe bursting open. My friend screamed and leapt at his pride and joy which was knocked to the ground as he started trying to bale handfuls of this stuff out with his hands.
Knocking the craft over allowed the still liquid and not yet fully expanded foam to flow to the other end of the canoe where it expanded and shattered that end as well.
A few seconds later and we had a canoe with two exploded ends, a mountain of solid foam about 4ft high growing out of the middle, and a chemist firmly embedded up to his armpits in it.
At this stage he discovered the reaction was exothermic and his hands and arms were getting very hot indeed. Running about in small circles in a confined space while glued to the remains of a fairly large canoe proved ineffective so he resorted to screaming a bit instead.
Fortunately a Kukri was to hand so I attacked the foam around his hands with some enthusiasm. The process was hindered by the noise he was making and the fact he was trying to escape while still attached to the canoe.
Eventually I managed to hack out a lump of foam still including most of his arms and hands. Unfortunately my tears of laughter were not helping as they accelerated the foam setting.
Seeking medical help was obviously out of the question, the embarrassment of having to explain his occupation (Chief Research Chemist at a major petrochemical organisation) would simply never have been lived down. Several hours and much acrimony later we had removed sufficient foam (and much hair) to allow him to move again. However he still looked something like a failed audition for Quasimodo with red burns on his arms and expanded blobs of foam sticking everywhere. My comment that the scalding simple made the hairs the foam was sticking to come out easier was not met with the enthusiasm I felt it deserved.
I forgot to add that in retrospect rather unwisely he had set out to do this deed in the hallway of his house (the only place he later explained with sufficient headroom for the canoe - achieved by poking it up the stairwell.
Having extricated him we now were faced with the problem of a canoe construction kit embedded in a still gurgling block of foam which was now irrevocably bonded to the hall and stairs carpet as well as several banister rails and quite a lot of wallpaper.
At this point his wife and her mother came back from shopping......
Oh yes - and he had been wearing the pullover Mum in law had knitted him for his birthday the week before.