First and foremost - when you are on the boat space and movement is limited. In the usual land held event have the photographer most of the time option to move back and forth when capturing the event from different angles and perspectives.
Photographer on the boat event doesn't have the luxury of moving anywhere he wants. When you are squeezed with your camera to the edge of the boat and one more step further means going overboard, you have to be prepared to solve this limitation and get the shots you are expected to capture.
Another factor which limits the photographer on the boat is the speed of movement. There may be something important happening on the other side of the boat and you have a good intention to capture it but rough water waves or high volume traffic on tight boat can slow you down in getting to the good spot for photography.
This is one of the reasons why we always work together. Having two photographers will eliminate the risk of not being in the right time at the right spot in half.
Conclusion - when you are planing your next event on the boat or yacht, don't forget to take in the consideration challenges your photographer will face when capturing your event for you.