A different take....
If the land owner asked you to get your property off his property, you should have done so. (It's not clear how he is your landloard if the pier/slip are leased from the city). You can't just go dumping stuff (or storing) stuff on other people's property. Apparently this has become a sore spot for both of you. By your account, the value is only $250, so before you get hot and start to file police reports and taking people to court, think a bit about unintended cosequences. If you get into a position where you have to get lawyers, courts, etc. involved, you are going to spend more than $250 if not careful. If the dinghy was on his property and he asked you to move it, why didn't you move it.