Sealing passages and voids are the duty of pour in foam - there are a number of venders of USCG approved closed cell foam and they all (that I've used) seem about the same, excellent.
I foamed 7 floatation chambers on my older Gemini
3400 - 3 were full when I inspected them, the others were leaking from the hull/deck joint of various cleats
When I sold her 4 years later, the chambers were bone dry, the boat waterline was about 1/12" higher (to the good), and she seemed somewhat quieter.
Used properly, pour in foam is very neat, no cleanup required, just pour in very small amounts - estimating about 2/3 the size of the void. Take your time, it is easy to do, and the stuff is very, very strong - stiff sort of strong. One area that had oil
canned from a bad haulout poppout back into shape and was solid and strong to a ball pean hammer tap.
Think Boston Whaler
Foam in Place foam (trade:Great Stuff) is good for emergency
repair of your dinghy
(I sewed and foamed a 18" long nail tear - held for years - but it is not suitable for structural repairs
like the pouring stuff.
I carry two cans - at least I did when I had a sailboat. One day I hit a coral head
in the channel at spinnaker
speed. Boom. The boat was taking on water, but after all, she had a spare hull
. I jammed a couple of wash clothes into the hole so she dropped to bilge pump
speed, and took a boat cushion and two cans of foam. Foamed the hole area, slammed a boat cushion on the foam and sat there for 20 minutes. Foamed the second can, let it set up with was paper covering the cushion and new foam, and sailed off.
Six months later, I pulled her and repaired the hole at the next regular bottom job.
This stuff could save your boat.