Jet ski is fiberglass. You could possibly get someone to take it free if the engine, trailer, or other parts are good, but old junk fiberglass boats and skis aren't worth anything as scrap, and are usually difficult to get rid of unless you pay to have them removed. Cars usually need a title to be scrapped unless it's old enough, but cutting it up makes it no longer legally a vehicle and the sections can be scrapped without paperwork.

This is all a legal grey area since the legal owner is out of contact. Perhaps call the police non emergency number and ask about abandoned property laws. You don't need the owner suddenly wanting their vehicles back a week after they're gone forever.