The fountain has stopped.  This is good because it means you can go for a paddle without getting your hair wet.

But once you step into the pool you can’t easily get out again.

Waves of monsters attack.  If any of them reaches the fountain then you’re dead, so each and every monster needs shooting.

This can take a bit of practice.

Try to learn the routes that the monsters follow.  In particular, when the monsters move in from the outer loop to the inner loop, they are especially easy to hit, so be ready to pick them off then.  Also, the monsters are killable as soon as you hear them emerge, so don’t wait until you can see them before shooting.

If you outlive all of the monsters then you’re free to leave the fountain.  Go through the door that appears in the top-left wall and claim your reward.