That's a tough question to answer. In my 40+ years of boating I can't recall actually moving someone elses boat to make room for my own. Sometimes it becomes necessary to "Adjust" someones docklines, say, in a case where you are sharing the same cleat on a dock.
If someones boat is docked in such a way, near mine, so as to present a danger to my boat or someone elses boat and the owner is nowhere in sight, I might again "Adjust" dock lines to prevent the other boat from hitting mine or someone elses. I'd say it's a judgment call each time.
NB