Here's what I have in mind:
• You start as aquatic-only race with a colony in the middle of the ocean. Your units and heroes have great movement in the water, but can't go on land.
• Terramorphing option in the research tree that allows to flood coastal sectors and expand oceans. Instead of roads they use rivers/ocean currents.
• You can research "landwalking" the same way as other factions research ocean travel via ships, except your units and heroes gain the ability to go on land — either by foot or in giant portable watertanks (either mechanical or organic).
• Units consist of several core race "specialists" and various aquatic creatures, think marine biologists and their lovely pets.
• Weapon-wise they could use Biological/Arc (masters of sea storms) or Biological/Psionic (but then we already have Kir'ko with the same set). Not sure about this.
I think Aquatics/Tidal is more likely to be a secret tech, for two reasons:
1) Oceans need some expansion to make more interesting in general. More hazards, more variation in biomes applications. Jungle islands, small mountains/oceans, maybe some more amphibious wildlife/maurader units. Doing all that just for one race to benefit seems extreme. As a Tech, all races get diver units, and you can get more piratical heroes and customization options across the board.
2) Flooding sectors and drowning planets just seems like quality doomsday tech material.
I guess it doesn't have to be one or the other, but I don't want an aquatic race with a tidal tech that is so synergistic as to cause other combos of them to be detrimental. And it might be weird to see watery sharkmen or Carcharozons or whatever with Promethean incineration tech.
Either way, I like the ocean start, but I'd make it coastal start, with tech progression allowing deep ocean settlement later. I like rivers as roads, but not rivers instead of roads.
As a race, having to research "Landwalking" seems like it might be a reasonably interesting mechanic, but it could also be very restrictive and annoying if the ocean doesn't get updated. I'd suggest that maybe armies don't regen on land, except wetlands and river lands, while arid and volcanic, and maybe arctic zones cause attrition or (additional) happiness penalties that could be researched away. As far as damage types go, Arc and Bio are arguably suited to an aquatic race. Large bodies of water and current might negatively affect the range firearms, lasers, electrical, and poison attacks, so if the race used them, they'd probably need to be a melee faction. Explosives only have one race using the tree, and are arguably more effective underwater than above it (concussive and depth charges), so I'd like to see the race have that branch. Psionics is likely unaffected by water, and we could see plenty of nautically themed psychic powers, but then we might be trampling psi-fish territory. (I just realized their appaerance is based on the xenos from The Abyss.)
As a Tech... Well, Promethean Plasma is it's own weapon-type, despite causing burning, and doesn't work well with most weapon mods. Tidal/Grav could be it's own type as well, although I admit I prefer more synergy between base weapons and secret tech.