When we were getting SV Watermelon ready to go cruising, Peter rewired the entire boat. It had 220V, he rewired everything to 110V - because you can switch back to 220V with just a few changes to the system, but you can't put 110V through wires intended for 220V. So, we were "good to go" for the first few years of cruising in the Caribbean and South America (primarily Venezuela, which had 110V and was about the only place where we ever stayed in a marina). For the one or two times we were in a Caribbean marina with 220V, we had a small transformer that we carried between the boat and our apartment on the French side of St. Martin (we were either on the boat cruising around, or living in the apartment during high season and friends and family from the States were visiting).
We first had a problem when we headed for the Panama Canal and stopped in Curacao to have our boat hauled to have the bottom painted and a few changes made. Curacao back then apparently had really screwy and unreliable power supplies, with the voltage fluctuating in various places from, if I remember correctly, 105V to 127V. However, the local hardware stores had transformers that could handle the multiple voltages that we found on the island. It was a relatively small transformer with various "plugs" that one used to set the correct voltage input (these "plugs" looked somewhat like electrical staples), but we had very few AC needs.
Carrying a multimeter as you cruise is very helpful - we sure got to use the voltmeter function a lot in Curacao.
That transformer worked very well for us until we finally accepted, about halfway across the S. Pacific, that we weren't going to see 120V anymore, and Peter changed the boat's wiring from 110/120 to 220/240V. Much easier. But we kept the transformer, "just in case".
Peter says that you can find all kinds of transformers, from simple plug-in power-out to those with dials to dial the correct voltages. I just did a quick internet search, and this is the first site I found - I know nothing about the company, but at least it gives you some idea of what is out there. voltage Converters, transformers