Check what sort of compressor it is, too. A small diesel-run job mounted on the prow of a small vessel, howling all day, can make your stay tedious. If you want the privacy of a twin or double cabin to share with your partner, confirm this at the time of booking, not when you are standing disappointed on the quayside, and about to board. Some vessels have patently too few toilets for the number of people on board. Others have en suite facilities but inadequate water supplies.
Some vessels have water-makers.
Find out if your bed linen is to be changed daily or weekly, and if towels are supplied. Also, ask what additional costs will be charged. Some boats charge for soft drinks and bottled water. Others supply it free. The same goes for beverages such as tea and coffee.