It's a mix for me. Until recently, I would take the boat to the dealer at the end of the season for oil change, winterization, and impeller (every 2 years or so for that). When the pandemic hit, so many people bought boats that the dealer was backed up a couple of months for winterization and, noticeably, repairs from the escapades of novice boat owners. So, I started doing the annual winterization myself. I did have one warranty item on my Malibu throttle control early on and that did cause a couple days loss of use.
In the past, I've been fortunate enough to have end of season work done at the dealer while I waited and watched - so no lost time or second trip to retrieve the boat afterward. That has also occasionally included some other sporadic maintenance like new riser gaskets, new plugs, new distributor cap. Once demand returns to normal, I'll probably go back to the dealer periodically just to have it checked over, but I'm doing more of my own work now.
I've done a few installation tasks, though. Full initial StarGazer installation on my MasterCraft years ago. Shower system, heater, and heated seat on my Malibu. Other than the heated seat install, no upholstery maintenance or replacement yet.