The original Polyurethane glue brand was Gorilla Glue. It thick honey like glue and is totally water proof. the use it properly you dampen one surface and use the glue on the other surface. The nice thing about it is that it expands to a foamy like look but gets rock hard. Many brands incl. Elmers now have it out cheaper than Gorilla.

The big downfall to it for boat building is that it's expensive - way more than epoxy. It's thick viscosity also makes it only good for gluing wood pieces. So, boat builders who already need epoxy for glass wet-outs would have no real nee to purchase it. I ocassionally use if when I don't feel like mixing epoxy to bond something that's going to be submerged. It would probably be excellent for a paddle - although epoxy might do the job better? Plus you can coat with epoxy.