I'll leave you to do the bulk of research on FOAM wrt Boats for yourself. You can look at the forums on iboats. Use the search on that web site with the keyword of "foam". You could read for years...
One comment on that web site says "For "approved" floatation, use pourable polyurethane foam from iboats or elsewhere... that's what it's for". Since it was found on the internet it is of course true.
I found other comments that say Great Suff is not the correct type. What is definitely true "required by USCG for all vessels under 21', it may be structural in your boat or just give you piece of mind". There are calculations for figuring how much foam you need and etc but sounds like you are replacing what was there to begin with.
Here is a tread on iboats about pouring foam: