Most large hotels in Marrakech have swimming pools that are open to the public. A very nice place is "Le jardin de la medina," which is located in the Kasbah in walking distance from Riad LakLak. For 250/350 Dirham you get lunch, which includes pool access for the entire day. It's very nice!
Two other options to relax at a pool in walking distance from Riad LakLak are the Club Med at the Jemaa el Fna place or the famous Hotel La Mamounia (expensive). The Mamounia hotel is also very nice for just having a tea or a Martini.
Our favorite spot outside of Marrakech is the "Beldi Country Club." It is only 6km from the center of Marrakech and easily reachable by taxi. You can relax under the shade of olive trees and use their pool. They have a tariff to use the pool only (200 Dirham) or you can opt for the lunch tariff (350Dirham), which includes pool access. The food is prepared fresh and very tasty.
Another great option is the Oasiria Water Park. It is an excellent spot for spending a day of fun and relax with the kids and for enjoying together the wonderful views, the delicious food and the variety of attractions.