Naturist swimming in winter at Vera Playa

There's a Sports Centre in Mojacar Village. 

There's a 6 lane 25m swimming pool thats heated to 29°C.

Not certain if any of the naturist areas have heated swimming pools.  If you use the one in Puerto Rey Sports centre which is outdoor saltwater chlorinated and heated, you will of course have to wear swimwear.

There are council run pools in Mojácar pueblo, Vera pueblo and Cuevas de Almanzora pueblo. All are 25m long and are laned. I'd recommend the ones in Vera and Cuevas in preference to Mojácar - simply because they are open all day ( Mojácar closes in the middle of the day) for longer hours and they have separate smaller pools for classes such as aqua aerobics which leaves the main pool free for people who wish to swim lanes. The pool in  Mojácar pueblo doesn't have this separate facility so when classes are on, 2, and sometimes 3, of the main lanes get taken over and the remaining lanes can become very crowded. Also, whilst all these  pools are heated, Mojácar pool is so warm it can actually become uncomfortably hot to swim in. In all the council pools costumes and swimming caps are obligatory for all users.  

One of the naturist urbanisations near the beach (you can walk naked from the residences to the beach) had a heated pool and an outside pool. I saw it a few years ago when I was home hunting. The monthly fees were twice the average and I remember this because the monthly fees where I am are even more expensive for no apparent reason. :)  - Our pool is open four months max.

Whether you could use it for a fee I do not know but it might be worth making friends with someone down there?

Looking at a map, it could be Edificio El Cano II?