Electrical: 110v 60 Hz, 220v 50 Hz and 12v for 12v DC for electronics while you run 24v dc for the rest. Networking radars, GPS, AIS, plotters has become incredibly complex.
Sewing: having a commercial sewing machine has proven to be useful (to make friends). Repair sails, awning, various canvas
Digital media: most popular boats have satellite com on board and produce one quality youtube video every week. Photos, blogs, manage fans on Instagram. You were hoping to get away from the hustle? You will be back in it quickly.
Refrigeration skills: get yourself a set of gauges ($50), vacuum pump, several cans of refrigerant, soldering equipment. Whomever can produce cold (freezer, cold beers or ice cubes) is king in the tropics.
I can add to this list. The world of handymen is incredibly unskilled. Come with a mechanical engineering or EE/CS background, you will be making friends faster than you can think.