Congratulations on the new boat! From what I understand you are talking about the lock that goes on the top of the trailer tongue in order to keep someone from being able to detach the trailer from the tow ball...? If the lock is too short, I would recommend making a trip to the closest hardware store and looking for a longer lock that will fit. I prefer a combination lock so I don't have to worry about losing the key. If you cant find a lock that suits your needs you can always take one of the safety chains that connects the trailer to the car and padlock the chain to the car.
On another note, If you do end up finding a lock for the tongue, make sure you have a locking pin that secures your trailer hitch to your car. I have heard of people who have had their trailer locked to the hitch but not the hitch to the car so they just unplugged the hitch and off they went!