From Under The Ocean Wiki
The primary controls are the WASD keys (or alternatively the arrow keys), which are used to move the player; and the mouse, which is used to craft, build, and interact with items and the environment.
|Left click||Use selected item on target|
|Right click||Enter crafting mode with target|
|PgUp/PgDn/Scroll Wheel||Change material layer|
|Spacebar||Switch to building mode|
|1-9||Select item in slot 1-9|
|Alt||Display item names|