unusual not to have seen at least a swamp by now. But not impossible. Did you circle all the islands you visited entirely?
-> mountains are never on the shore of ocean. They don't even usually get close to rivers either, once in a while on that.
so you should see some plains (brown, large tall rocks size of a big city building) and some swamp (dead looking trees ) soon.

got a seed? We can steer you if you don't want to spoil it but need a little help.