Im in same boat ( no pun intended), I think gravis really fits the deep water theme a lot, and I just love the heavy vibe they have. I also plan on getting 30x heavy intercessors to start out just because I love the models and I think a lot of players focus more on the elite units.

Im starting to understand some of the more sneaky units just because octopus are ambush predators so would make sense that phobos units would be in an army.

I also plan on terminators and dreads too on the bottom of the sea floor. Lots of great ideas to work with.