Sad to say but you can't stop anything being stolen if some one really wants it. You probably already know this.
On my RIB I have Three wheel locks on two wheels. A chain through one of the wheels. A built in hitchlock, I try and always leave a car in front of it on the drive and because I'm a shift worker somebody is around most of the time. My greatest worry is some scrote knifing it whilst its at the house.
When I'm using it. I worry about coming back to no trailer and when its moored up I don't go too far from it and keep taking a walk back to it if its out of site. My mate has a Krooklock for his steering wheel on the boat. Only person I know who does it but its a good idea.
My next security item is going to be CCTV. Thats become quite cheap now. Other than that I pray and make sure I meet the minimum requirements of my insurance. (Fingers crossed)