The construction of the sewage treatment plant in Nerja has been delayed several times and it will probably not be completed until next year at the earliest.
http://www.surinenglish.com/local/201802/23/work-restarts-nerja-sewage-20180223093555-v.html
People debate the water quality and safety on several online forums with mixed opinions. Some return to Nerja year after year for the beaches and swimming; others don’t.
There is a Blue Flag system that awards blue flags to beaches based on cleanliness. The closest to Nerja is
Ferrara
Torrox
Spain
Blue Flag season:
15 Jun - 15 Sep
Map coordinates:
36.7297133,-3.9643596999999317
Full details
Display all beach information
http://www.blueflag.global/
http://www.blueflag.global/show-site?siteId=7954
One forum user recommended: “The Carihuela area of Torremolinos is 20 minutes by taxi from the airport and a lot shorter time by train. Torremolinos is a very popular resort with the Spanish and this area especially so. The beaches in Torremolinos are more sandy then in Nerja and can be accessed in all parts from a flat paseo, no hills to go down.” This area is near Malaga.
https://www.tripadvisor.com/ShowTopic-g315917-i5571-k8114189-o10-Sewage_Treatment_Plant-Nerja_Costa_del_Sol_Province_of_Malaga_Andalucia.html