Charter boats see a lot of use over a short time. Things wear out, fall overboard, break,etc. and the older the boat is, the more problems you'll have. There's a reason they charge more for the newer boats.
We've chartered in Caribbean every year for the past dozen years or more and will not charter a boat that is 5 years old again. Even in the better kept fleets, a 2 yr old boat is entering middle age. Next time you charter, don't hesitate to contact the charter base with any kind of problem. The better ones will send out a chase boat for problems like batteries not holding a charge, or refrigeration not working. (Been there, done that, on more than one occasion). It's also a good idea to have a cell phone that works in the area--except for shadowing by the volcanic terrain in places. Make sure you have the base telephone number before you leave.
One of the more important things you can do is to go over absolutely everything before you leave the base. You may be tired from your trip down, but this is important. Make sure the stove works and stays lighted. The refrigeration should be on before you come aboard and you may find a block of ice in the icebox. If not, put one there for your peace of mind, just in case. We've found that a malfunctioning stove or refrigerator is not grounds for a charter adjustment, according to the contract language, but it doesn't hurt to ask for consideration if a problem took the edge off your enjoyment of "paradise".
The best advice is to go with a reputable operation and select a boat that is no more than 2 years old. It's worth the extra money.