I would suggest that a good starting point is your insurance cover . ( Think vehicle and/or trailer breakdown in transit - roadside repair may be impossible so do do you need recovery ? ) . Good trailer servicing is a must if you you are travelling regularly !
As a minimum , if your insurance cover stipulates that must have a wheel clamp , then that's your starting point . Personally I use hitch locks and additional chains to ground anchors when at home . Do a search on trackers , micro dots and also outboard security bolts ( which may again be an insurance requirement ). Out of sight is good !
Further searches will show that many of these devices will not stop a gang coming along with a power saw , and hacking off half your transom in order to nick your outboard .
Common sense and vigilance at service stations is also a must .
I'm sure others will be along shortly to depress you even further