Don't know about 525 but La Serena Hotel has a small (about 10-15 metre) indoor heated pool which forms part of its Spa. It's open Tuesday to Saturday from 4.30 to 9.00 pm and Saturday and Sunday mornings from 10.30 to 1.30 I think. Closed Mondays. It cost €10 for one entry which includes pool, gym, sauna, thermal and hot & cold spas. However for €50 you can have unlimited use for one month or €30 for 3 sessions per week for one month. There are of course other spa treatments, massages, etc available at extra charges.
There are also large indoor heated pools in Los Alcazares, Torre Pacheco and Pilar de Horada where you can usually pay per swim or buy a book of tickets.
I'm an avid swimmer and have started using La Serena but it is rather small and very, very warm. Haven't yet tried the public pools.