Bluetooth speakers are great but they do have some points to consider.
1. They work on 2.4GHz. Not too bad (certainly not as bad as the old days of 27/40MHz) until you have a lake full of models running on 2.4GHz and 20 speakers running on 2.4GHz!
2. Range is limited - Severely. 6-7M sounds a lot, but that's line of sight, and it's not exactly far away for a model, particularly a larger scale one.
3. When you lose connection, it can take forever to reconnect. And every time your model goes out of range, it will take you 10,20,30 seconds to reconnect, possibly manually.
4. Bluetooth audio streaming suffers from the issue of digital noise. Rather than go a little fuzzy, it will simply cut out, jump, and make odd noises.
None of which lend themselves very much to model boating. It's possible to use one, but it'd probably be a lot better to have the phone onboard somewhere - which kind of negates the need for bluetooth.
They are good when used 'normally' though - I can tell you from experience that a Bose Soundlink Mini will fill the entire engine room ampitheatre of HMS Scott with sound - and that really is a big achievement. That's quite an expensive speaker though, but a fairly cheap one will still perform very well so long as you stay relatively close to it.