How many photos do I need?
At least five of the boat, one of which is set as the cover. More is better, and photographs of the actual vessel matter far more than stock imagery.
Five is the floor, not the target. The cover photograph is the one carrying your listing in search results, so it deserves the most thought.
Photographs of somebody else's boat, or generic stock imagery, will not pass review. Guests book what they can see, and they notice.