Cistus 'rock rose' - will it drop leaves in pool?

Cictus will drop individual leaves as new leaves appear. This happens throughout the year but mostly in the spring with a rush of new growth. This is true with any evergreen plant. Even cactus (which you definitely don't want right next to a swimming pool) will occasionally drop a pad. I don't think there is such a thing as a litter-free plant.

Furthermore, Cistus prefers a dry environment. It will not survive if it is repeatedly wet by splashes from the pool.

I've just been out and had a look at mine - they are at least 20+ years old and rarely pruned. There is a lot of leaf litter under them but then, as I say thay are not pruned often and are big so there is a lot of litter.

I would say that if you do keep them well pruned to keep encouraging new growth, they would be relatively leaf litter free but then that would also encourage more flowers so the dropping flowers could be a bit of an issue.