Hi Josh -
No guitar shops on the island that I know of - you might find a fellow musician who wants to jam tho.
Water taxi is often like a very crowded bus - the case of beer concept won't work. Also note the downside of drinking beer on a 90 minute boat ride with no rest-room; and no-place out of the wind or public view to relieve yourself. I've observed some really miserable young men who had a few pints prior to getting on the boat --- leave it to your imagination. Strongly recommend arriving via airplane - it's easy & pretty. If you feel inclined to take the longer route back to the airport at the end of your trip you can opt for that, but don't do it to yourself on arrival.
Enjoy every minute of your stay. Welcome to the island!!