Cool!!
I see that you didn´t mount the speakers permanently, I appreciate that.
You could couple a generator to a paddlingmachine to drive the amplifier.
May I suggest a couple of suitable music pieces.
The first is "The Stripper" David Rose played by Henry Mancini.
The second is "Riding the waves" from the soundtrack to "The Deliverance".
The third is "Cool Water" by Burl Ives.
And a lot more titles that have boat and water themes.

Richard K.