I think there will still be islands you can not walk to, these may have resources on them, but given the city qty limit this isn't any big advantage now anyway. And yes, you could 'raise land' across the open water and walk to the islands. In previous versions I usually build a few ships to scout for islands with resources and they did exist so I assume they still do. Alternately if the edge of the island was in your zone of control you could teleport there.
