Well, Shadow, there are a lot of options open for you:
- Uncolonized land regions
- Unexplored land regions (requires QuintonBeck's approval and cooperation)
- Underwater regions (unexplored or not)
- Underground regions (many of which are unexplored, but 76B and 21B are still not occupied)
- Taking over an NPC nation

For polar regions, there are probably anywhere up to 8 polar regions in the south yet unexplored, and I think P6 up north is also still unoccupied. Not sure about P6 though, but the south is wide open.

As to underwater vassals, you'd have to ask WaylanderX or RandoMan about that, as they are the only two players who can vassalize others and who already have underwater regions.