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. Some do not even have a working compass.
Helmsmen often use coastal landmarks or simply follow other vessels.
If you hope to dive the more remote offshore sites, like the wreck of the Thistlegorm, check that your vessel has the means to find them. Emergency equipment may be carried, but this does not mean it will work. This is extremely important in the case of emergency oxygen for divers. Ask to see the O2 equipment when you first get on board. Insurance and licences are important too.