Seriously, this is not the place to use threats much less legal
action. People take care of people they like. This is not just the schedule but how carefully the job is done. You also have to be realistic about the boat yard business at Memorial Day. Too many boats and too little time.
It costs very little to be a favorite customer. For some reason, a lot of boat owners just turn into jerks when dealing with their yard.
Ask about their kids
or whether they own a boat ("Nice boat!"). Let them give you some advice about taking care of your boat (which will probably be very good). Compliment them on good work. Tell the boss when one of the workers is doing a good job.
With a little effort, don't be surprised if your boat mysteriously jumps ahead of others on the travellift schedule and a few things get left off your bill.