So here's the deal. When the laser beam passes from air into water, refraction causes the beam to bend away from the pool floor. A large amount of laser light will be reflected in random directions at the air-water interface from pool waves.. This results in a mess. It's generally just not done because a fully underwater laser projector is not cost effective nor off the shelf.
Unless you have a very deep pool, it's not a great idea. Consider a water screen instead, but make sure pool guests cannot be sucked into the very large pump required.
Sorry
Steve
Qui habet Christos, habet Vitam!
I should have rented the space under my name for advertising.
When I still could have...