I'd say you need a "feel". Kinda like combination of gameplay, mechanics, visual style.
Look at the Starcraft Zerg for instance - they FEEL like a numberless devouring horde.
Typical mapmaker takes a bunch of Naga models, puts them together, and has a custom race that fits together visually but is really uninspiring. They don't FEEL like an underwater horrors race, even if you give them giant squid monsters and stuff.
Now imagine you gave them ability reveal any area of the sea and summon horrible monsters there - but only in the sea. Plus, say, an ability to create temporary lakes (terrain deformation + water model + pathing modifier). Enemies stay away from the shores. Enemies see lakes forming and run away or bring ranged heavy-hitters. That would be closer to capturing the feel, even if it would be situational and comparatively weak.