Deco Cement, if you can repair a cessna wing with it, you can use it for a raft
Other then that Plastic Weld or a Good Epoxy
There is an epoxy, I forget what, that can seal and fix pvc pipes, I know I used it at my last place, a townhouse I was renting, worked like a charm
Also glued my buddy's mirror back on the his passenger side of his grand prix before he traded it in

I think that the plastic weld would work fairly well if used sparingly, but yes be careful. Have you considered using plastic gallon jugs, those work well, I have seen that done before.