"UE megabooms"
These seem quite neat.
https://www.ultimateears.com/en-gb/w.../megaboom.html
Bluetooth speakers, integrated rechargable battery.
Bluetooth or cable to one and you can link more speakers by wireless (mentions 150+).
Are waterproof "can be immersed in 1 meter of water for up to 30 minutes" but I can't see where the web site discusses the sound quality under that circumstance.