Hello,

Yes - this product can certainly be used underneath the water line as well as above.

Please let me know if you have any other questions!

I'd be very careful about the use around the pool waterline. You'll be picking them up off the bottom.
Often this could need brushing to clean pool "flotsum" and only the genreous use of epoxy setting materials will hold them. They tend to have varying amounts of adhesive
adhering them to the mesh making them easily loosened.