Thanks for the comment!
In each game there are 2 enemy tribes, so it looks like in your case one of them has been spawned on an unreachable cluster of islands, sorry. I considered such situation too unlikely to check for it, but it seems I was wrong.
Yes, there is a win condition: both your opponents have to lose all their islands (you don't need to colonize every island). You will be shown a clumsy "Victory" popup :)