Most the marinas that I choose to spend time at have no problem with "stuff on board". Myself, I have no problem with excess stuff on board other people's boats.
I think this has more to do with what sort of stuff, and how it's stowed. Nobody is going to have a problem with boating-related gear stowed tidily on deck. Other, unrelated stuff, strewn about or overflowing onto the dock/finger will probably draw complaints.
I've left a kayak and my dinghy up on the foredeck a few times with no problems, but the marina staff commented as soon as I forgot about a cooler that I left on the dock. Same cooler stashed in my cockpit, no problem.
Most marinas around here simply want to maintain a presentable appearance. That's why a lot of them require a photo of your boat when you apply for a slip. Some of the houseboats in those videos would not likely be allowed in the marina I'm in for this reason. I wouldn't mind seeing them, myself, but it's not my marina.
At the same time, as long as you aren't disturbing anyone or making an uncontained mess, my marina doesn't have a problem with anyone doing work projects on their boats.