Simply him to try living on a boat for 3-6 months first. You can say “it will help you determine what kind of boat you want.”
He and his wife will notice a thousand little changes, inconveniences, and maybe even some advantages he never expected.
Most likely, like children, having experienced it, they will move on to some other fantasy/obsession.
Double bump for renting a boat and try it first. It is not for everyone. Lots of folks underestimate the cost to maintain a boat, marina costs, etc.
Yep, go to either the San Juan Islands or the Caribbean and rent a boat for 1 to 2 weeks. If they like it then shop for a boat. If they find out this is not for them they are only out the rental money. Much cheaper than trying to sell a boat (they would never get close to their purchase purchase price back).