You are misunderstanding the section. attraction=boat_ride is for small boats viewing a city, or cable-propelled in theme parks. It’s not for ferry class ships, or cruise ships. The page itself Tag:attraction=boat_ride - OpenStreetMap Wiki is clearer.
As mentioned by others, cruise ships are not added due to irregularity. But regular sightseeing ship routes should be acceptable. They serve dual function of views and transport, similar to open-top double-decker bus circular routes.