When I got my rig 16 years ago I studied these boards for many things, one of which was trailer security. I came to the conclusion that you won't stop a professional thief, but how many of those are roaming the boat launches looking for trailer? I decided to put locks on the receiver and hitch to stop the opportunists and haven't had a problem.
That is until yesterday. While I was on the water someone tried to steal my trailer. This wasn't a pro or it would have been gone unless they had gotten the other trailer they messed with first. I was at a well used ramp but launched at first light when nobody else was around. The other boat had launched just after dawn. On both trailers the locks worked, but on mine they tried to disassemble the swing away tongue. They were able to remove the hitch pin but didn't have the correct wrench for the big bolt. Luckily I had a spare bolt and was able to get home.
Now I am rethinking this whole subject. Locks on the wheels, lock on the hitch pin, lock on the Drotto? I wasn't overly worried before since my truck has an alarm system that will go off if the trailer is unplugged when the truck is locked but that won't stop them from getting the peripherals.
How far do you go to protect yourself from thieves? I had some very scary thoughts about the hassle of getting back to the ramp and not having a trailer (or it being unusable.)