I would like to plant a row of the evergreen Cistus 'rock rose' along the
swimming pool. I am not sure if this plant is mostly litter-free. Does it
drop leaves anytime like when old growth turns brown? Is the only litter
from this plant the pink flowers dropping in the summer?

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.