Swimming

Water Boosts
Pressing jump underwater gives a speed boost but reduces breath.

The amount of speed given reduces each time, and more than a couple will quickly cause slugcat to start drowning.

Using boosts to escape danger
Speed-boosts are usually best saved for dangerous situations (eg. escaping from salamanders)

Fast horizontal water movement
Dipping under the surface and boosting once horizontally and slightly down, then resurfacing and repeating is an effective way to travel quickly through large surface bodies of water. This makes use of boosting, using some breath, but preserves most in case of creature attacks.

Buoyancy
While swimming, slugcat has a constant upwards drift towards the surface of the water. To swim underneath the surface (and consequently faster), use swim-strokes / boosts to swim slightly upwards, angle down slightly to maintain initial depth, and repeat,

Counteracting upwards drift is important for swimming through underwater tunnels, to avoid being trapped against the ceiling.

For swimming vertically, moving straight up makes the best use of natural buoyancy!

Wriggling
Alternating between up and down while swimming horizontally also increases swimming speed. This is important when navigating underwater tunnels.