Compared to other core regions or biomes, underwater regions are far less homogenous. The ecosystems found beneath the surface vary tremendously based on the depth of the water, the light penetration, and the temperature. Designers creating fictional underwater areas are not crafting a single monolithic region – but many. Each subarea, from deep sea vents to kelp forests, are unique and each subarea is an opportunity to establish different tones, creatures, plants, and factions (if the underwater region is inhabited).

In general, shallow areas are often depicted as less threatening or safer areas. The availability of light helps establish a more friendly tone and the bright colors of locations like coral reefs aids in this. In contrast, deeper areas, where the light fades and large things could be circling, are often seen as more dangerous and horrifying.