I have been involved in installing them inside vessels above the waterline. I used "ordinary" APs like the MR36.
Things to consider:
* What the vessel is made from (metal, wood, fibreglass, etc.). Water will completely absorb the signal.
* There are high RF devices, like RADAR - stay well away from them.
* Some areas may contain dangerous chemicals or high levels of heat or cold.
* You will need considerable protection if operated where salt air can get in. I would put the APs into marine grade rated enclosures.
You may need to implement a "safe to operate" style system. This may mean you have to provide a way to "lock out" the power to the system to allow maintenance of the vessel.