Difference between finishing a wood deck and finishing a boat?

Because they will peel heavily and become a maintenance nightmare. You do not want a filming type varnish on a horizontal wood deck.

So then boat decks are a maintenance nightmare? Why would boaters use film finishes?

Water and salt protection and they do not put it typically on walking surfaces. Mostly vertical.

I think the latter statement is doubtful. Do a search on Google and every site discussing wood boat decks recommends multi-coat film-forming finishes - irrespective of marine or freshwater.

However your explanation about maintenance is probably correct - as a group, boat owners are probably willing to spend a lot more resources on their boat than on a house deck, and most wooden boat decks are considerably smaller than wood house decks.

Have you ever been on a cruise ship? The railings have a marine varnish that is constantly maintained while the flooring is allowed to naturally gray and patina without any coatings at all. I would think it would be extremely slippery and dangerous as well if the flooring had a filming marine finish.

I can't recall how the cruise ship looked, but I can certainly imagine it that way. But there aren't a lot of discussions on the internet about how to refinish your cruise ship. I was thinking more about small sailboats - 23 foot, etc.'s

Here's a Google search for: wood boat deck finish
> https://www.google.com/search?q=wood+boat+deck+finish

I maintain that multi-coat film-forming finishes are the standard for wood decks on recreational boats. Anyway, I accept that the standard practice for house deck finishes is motivated largely by a greater weight given to ease/cost of application and maintenance over longevity, along with certain preconceptions about the desirable appearance (film/no film) in each case.