I'm still waiting for a Pokémon game that actually has "too much water" but executed well.
Imagine a deep-sea area bigger than the diving locations of Hoenn.
Sure, there would be the good old generic Water types in there as well, but with the underwater locations spanning many routes, there'd be a lot of room for diversity, expanding the list of aquatic Pokemon with all kinds of non-Water types.
I can imagine pure types of Ground, Grass, Poison, Dark, Electric, Rock and others appearing easily, or any combination of them.
A Poison/Electric jellyfish, a Ground starfish, a Fairy type mermaid-inspired design, a Dragon/Grass algae seahorse, a Steel sea-mine, a Ghost type ship-wreckage based creature, etc...
