In most cases the skipper sleeps in the saloon area. However, it is strongly recommended to charter a bigger boat and to provide the skipper with his own cabin. Since he is in charge of the day-to-day running of the yacht as well as the security of people on board, he should be well-rested - apart from that, the guest will have more comfort and a free saloon area to hang out.
Every vessel is registered up to a certain number of people (the law does not allow this number to be exceeded) - if this number does not include a berth in the saloon area, the skipper is required to have his own cabin.