Guitars on board is awesome. A friend of ours brought one along in a hard case during the commissioning trip and in other circumstances my wife brings her "Resonata" beater guitar (which I think was made in the GDR). It wasn't left on board for long periods of time, but we noticed another problem: it takes up about as much space as another passenger, though it does eat less.
The OP didn't say what the size of her boat was, but she did refer to it as "small"; ours is a 27'. With a guitar and all the sail bags and back packs, it was an interesting challenge to sleep four people on our boat (especially if you insist on having access to the head at night).
Challenging but worth the effort.