I see that you are from the Netherlands. Power supply on most LOBs will be whatever the local power is. In the Caribbean, that means most will have 110v 60hz electrical outlets just like in the USA and Canada. If you are coming from Europe, you will likely need adapters unless the boat also has 220v outlets. Check with the LOB before you leave home. If your LOB is in Asia or the Red Sea, then I do not know what they will have as far as power supplies go.
As for clothing, on most LOBs, fashion is not a premium, being dry and comfortable is. A couple pairs of shorts and a few t-shirts (especially the quick dry kind) will likely be all you need. Don't forget to bring a few swim suits so that you have a dry one when you really want it. Lycra/nylon socks have been mentioned, but a long sleeved rash guard can make getting in and out of a wetsuit easier especially by about the 3rd or 4th day when you start getting a bit tired.
The other thing that you should absolutely not forget is a sense of humour. Something will inevitable go wrong at some point during the trip. It is a lot better to laugh it off than to get angry about it.
