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. The fact that a vessel is thus equipped is neither a guarantee that it is adequate nor that it is actually working. Even things like electricity, which you might have taken for granted, can be a problem.
Some vessels have inadequate generators or run on low-voltage systems.
Check if you can recharge batteries. You may find that those needing a 12-hour charge are no use to you on a particular boat. If you value your photographs, you will probably prefer to take your unprocessed film home with you. I have yet to find ship-board processing up to the standards of consistency and cleanliness I demand. Surprisingly, many vessels carry little or no navigation equipment.