When Prosise originally posted about the speaker/metronome, I decided to try it out for myself. Accompanied by my wife, I launched my boat, took it out over 30', and physically dove in the water. I had my wife turn on my 12V thumper which I made myself and have been using for several years. I listened closely from about 8-9 feet below the surface while holding my breath.
I returned to the surface, had my wife turn off the thumper and turn on the Bluetooth speaker and metronome. I dove back down and listened again from the same depth.
I found that although both were audible, the thumper was much more audible. I should mentioned that I have a trowelled-on layer of non-slip "Safe-Floor" on my boat, which I'm sure dampens some sound, but, that dampening was the same for both devices.
Bottom line: I found the automatic thumper to produce a greater volume of sound which I believe penetrates the water deeper and further than the quieter noise produced by the speaker system.
Hope that helps you!