Simple mode :

Just wear your equipment
Jump into water...

Forward, Left, Right arrows for direction
PgUp and PgDown Keys (or move your mouse Up/Down in Mouse look) to ascent or dive
You can switch to Fly mode by clicking the "F" key on your keyboard.

Jacket, Tank, ... are resizable from 30cm to 3m and more, from very small avatars, kids, adults, to giants.