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. All charge for alcoholic drinks, save for the occasional free bottle of wine. Ask before you book. Air conditioning can be essential below decks during the hotter parts of the year, or in the southern Red Sea.