Sadly, vehicles left overnight at boat ramps have had their catalytic converters removed. Law enforcement believes that the thefts are to pay for drugs.
In response to statewide thefts of catalytic converters, the Maine legislature has recently passed a law, in force since Aug 8th 2022, which is intended to make it more difficult for thieves to sell, and scrap metal operations to buy, illegally removed catalytic converters. Illegal sale or purchase risks fines and jail time as class D and E crimes.) So perhaps this issue will diminish in time.
For now, it would be best to be dropped off for your trip, and then picked up. Looking forward, the PRPT will try to find safe places to park.