Thanks for the feedback!
You're right, the dive is a "booster" in code because it temporarily raises the velocity limit. It's called "Dive" because it provides a double boost downwards.
I had considered only providing left, right, and dive actions, but I discovered that made it too easy to accidentally swim into an obstacle if the player pushed themselves downward. Player movement has momentum underwater, and the best way to slow down is by pushing yourself in the opposite direction.
I'm glad you had fun!