A decent lock and good insurance and don't worry about it.
I use this plus a disk lock and that's probably about as secure as you can do as far as tongue locks are concerned:
If they want your boat they'll come prepared or possibly even just drag it onto a flatbed. It's pretty much not possible to defend against that without going to extremes.
Don't waste your money on this tongue lock from master lock: http://www.amazon.com/dp/B000IIFQ26/..._6QjUtb1YMWK3K
We had those on 10 trailers at work. They always seemed like flimsily junk and I figured I could just kick one off the tongue. Finally one day someone took too long to show up with a key. 3 kicks and it broke apart and came off. No tools needed.
You can get those cheap Chinese gps tracking modules off eBay that accept a prepaid SIM card and send you an SMS with the location if it moves if you want a little extra "insurance" so you don't have to worry as much about it. You could also get a camera that you can access from your mobile device.