It seems you can use this waterproof bluetooth speaker in the sea water / salt water without breaking the speaker.
But using Bluetooth technology to stream the music is probably a bad idea. The main reason is the range between the speaker and your bluetooth cell phone/iPhone must maintain in a close distant or the speaker won’t tune the music well. And who would like to bring their cell phone/iPhone to the Sea anyway?
But this Waterproof bluetooth speaker might works well with pool/bathtub environment.
(Price: $40 at Chinavasion)