I don’t know anything about DIY rafts, but are you thinking of TPO roofing? It’s heat sealable, available in different thicknesses, and tough, but I don’t know if that’s the stuff.
Vinyl coated polyester (VCP) is what the heaviest duty industrial tarps and many commercial inflatables are often made of and it can be attached to itself with vinyl cement - maybe heat sealable, but it doesn’t ring a bell, but maybe I just haven’t paid attention to that use.
Hapalon coated nylon is also used for weather resistant covers, tarps and commercial inflatables. I have no idea how it is attached to itself.