A well lit parking area is first on the list of how to keep the scummies away. Secondly google "motion alarms boat" and check out what is available.
As far as hitch locks they are easy to defeat if the scummy has the space and time. A battery powered side grinder with a 4" cut-off wheel will make quick work of almost any hitch lock. The key is not giving the scummy a quiet dark and secluded place to work. Lights and alarms.