....on a couple things:

Dog comfort level aboard the boat: Does the dog take the boat motion and inability to exercise in stride, or is it distressed?

Cruising style and locale: I would say that someone who likes to make frequent longer passages and/or cruise places with minimal shore opportunities is doing their larger dog a disservice. On the other hand, if the longest leg is normally a hop that is less than about 8 hours between places to go ashore, it probably isn't a big deal, especially if the dog can get lots of exercise for an hour or two, depending on the dog's condition and age. (more than just a few minute walk to do its business). For some dogs, a great way to get some exercise is to walk them while riding a bike at a speed that allows the dog to stretch its legs as it wants. Other places, the dog might be able to be turned off leash for a bit to run with no risk of the dog or residents....

Taking my kiddo with me up to the North Channel for a couple weeks worked out great! Short hops, lots of freedom to go ashore off leash, fresh water. He had a blast except for a couple of brief episodes of queasy motion sickness (no puking, but some salivating).