Guitar is the obvious choice, portable, not stupidly loud and an amazing array of online tutorials and the ability to play all styles of music. The choice of instrument is baffling tho, full size, 3/4, steel string, classical... Make sure to get a tuner.
I trained on the upright bass, no room on a boat for an instrument of that dimension, so I settled for an acoustic bass guitar, cheap instrument with quality strings gets over the line. If you go brass it will also cover the need for a sound signal