Look at Bose Revolve Plus, wireless, water resistant and hold a 16 hours charge and these little guys pump!
Get an old receiver from a garage sale or a new $99 Sherwood from Amazon. Buy a couple or few outdoor speakers. Buy as many "impedance matching volume controls" as you would like to control "zones." Buy an Amazon echo dot.
Use the impedance matching volume controls to balance the sound perfectly around your space. Use the Amazon echo to change volume and music while swimming. "Alexa, play my favorites on Spotify" "Alexa, volume up" "Alexa, pause"