instead of islands why not have other rafts. The other rafts could have people in them or have the raft as a wreck which you can loot for resources. The rafts with people you can either attack,offer resources or ignore them. If you attack you can loot their raft and attach their raft to your own. If you offer resources you can either trade or befriend them. If you befriend them they will join you,your and their raft will attach. With this the goal of the game will be two things. Unite the rafters all into one raft or conquer the other rafters to become the strongest rafter.