Interesting ideas for making an island John, definitely agree on only using closed cell foam. If your island has any size to it, adding a lattice style inner frame of pvc would help keep the structure intact.
If your glueing anything to foam, 2 part epoxys are best, but are slow going when doing large area's, Gorilla glue is easier to work with waterproof and foam safe.
Looking forward to the construction of your island, remember photo's.
