I think boat maintenance schedules are skewed towards casual users who put maybe 50hrs per year on boats. Given the high rpms I generally figure an hour = 100mi in a truck. Probable less if pulling juniors and trick, probably more if pulling 36mph slalom and 35.5 jump.
I maintain a couple of club boats (one new MC and one 6L 196) plus my own 200 and sometimes the local college boat when I feel sorry for it. MC still recommends conventional 15-40 for the 6L but Nautique recommends synthetic 5-30 for the same. I change the oil every 50hrs in the new (under warranty) MC but the synthetic looks almost new at 50hrs, so I’ve taken to letting that slide to 75-100hrs. However we put 20-30 hours per WEEK on the boats, so they are getting serviced every 2-4 weeks either way.
Impellers still look new when changed every 200hrs if it has only been a couple of months. If we put 50hrs per year on the boats, I would change impellers every 50 hours (annually) but see no need to change $50 impellers every other week when they don’t sit and degrade. I change tranny fluid every other oil change and fuel filters every third oil change (how often do you change tranny fluid or fuel filters on your vehicles compared to oil?).
I change the spark plugs once a year (300-400hrs) but I doubt that is actually needed. I keep spare belts handy to change when one starts to look worn, but don't change them solely based on a certain number of hours. My 200 has 920hrs and the belt still looks new, so I have never changed it.
Point being, necessarily maintenance is a function of both time and hours, other than oil, which is truly just a function of how many engine revolutions.