We have been on many gulet cruises and have never worried much about security. I tend to lock my wallet and anything else I might worry about in the suitcase and then forget about them. Towels have always been provided and the crew will give fresh ones if asked. A big part of the crew income comes from a tip at the end of the cruise and they will do anything reasonable to ensure you enjoy your holiday and leave a tip. I don't mean, though, that they are just out for a hatful of money - I have never met a nicer, more hospitable bunch, than the Turks.
I did lose my favourite sudoku propelling pencil on the last cruise though...............
A few people have talked about getting luggage to a hotel ready for their second week. I would never want to leave the tranquility of a gulet and move to a hotel! Do the hotel first - wear your posh frocks and designer stuff - then the second week you can stop worrying about keeping your clobber in good condition and just stuff it in a suitcase. The second week you just need shorts and tops.
On a slightly different topic - I spotted, when I got back home from our last gulet, that my digital camera charger and my mobile phone charger had the same voltage and the same connector. I could have saved the weight of one of them in my luggage. With the 15 or 20 KG limit every item counts. Every gulet I have ever been on offers mains electricity at some point in the day but it is not on all the time. I always leave something plugged in so I don't miss it when the power comes on. There are usually a few sockets in the bar area which are on all the time and the crew have always let me use them if something needed to be charged up - just ask.
Also if you go for two weeks you don't need to take two weeks worth of tops, underwear and shorts. They can be washed in the cabin and will dry out in no time up on the washing lines. If you intend buying a load of new stuff while you are there take even less with you and save the weight of bringing lots of your own old stuff back.
Finally, most gulets have a small library of books left behind by the people who went before you. But not at the very beginning of the season. They clear them out over winter. We went in May and the boat had been out for just one week by the time we arrived. Not a spare book to be seen anywhere.
Hope that helps.