Hi
On a basic gulet you will most probably find a socket by the captains (bridge?) where he steers it. He will be happy for you to use this when the generator is on. The deluxe gulets do have sockets in the room but again they only work when the generator is working. ie early evening for a couple of hours or when in the harbour. The deluxe gulets also have hairdryers!

You can also get gadgets which provide battery charging in an emergency, which might be worth considering. I'm sure if you did a search on Google or similar, you would find them. No idea how expensive they are though, or if they're suitable for all makes - you can get them to charge mobile phones in an emergency too.

I am 22 weeks away from going on my 17th gulet holiday. Definately go for the luxury gulets and there will be sockets in the cabins. Any that don't would be very old boats by now. As others have said, if the generator is on or if the boat is under motor power (usually twice a day) then you will have no problem charging anything. If your battery was to get low, the Captain will more than probably switch the generator on for you to charge up.