What are four ecosystems a scuba diver might experience?

The "Blue" pelagic surface marine

The "Dark" benthic marine

"Reef" corral reefs

"Lacustrine" lake ecosystem.

"Intertidal" shallow water marine.

There are many more

Tropical

Arctic

Temperate

The ecosystems of different seas/oceans

High salinity

High energy

Low energy

Coastal

Etc.