How to prevent boat theft?

Never leave the keys in the boat when unattended.
Consider removing the battery.
Park your boat at a safe location.
Install an alarm system.
Remove valuables belongings and expensive equipment
Secure your documents.
Make sure you record your HIN number and keep it in a safe place.
Paint the name on the transom of your boat.
Install a portable tracker.
Try to avoid making it stand out too much when it's not in use
Choose a safe marina.
If your boat is on a trailer, install a trailer hitch lock so the trailer can’t be towed or take one wheel off the trailer and store it in your towing vehicle.
Disable the boat when unused. If you must leave your boat unattended in the water for an extended time period, you might consider taking the fuel line from the portable tank with you.
Engrave it. Ensure that your boat, trailer, and electronics can be easily identified by engraving your hull identification number (HIN).

Use a chain and padlock, securing them through the vents of the wheel and around the trailer frame.
Use a wheel boot, similar to those employed by parking-enforcement agencies.
Tight up the trailer against a tree, fence, or retaining wall at your launch site.
Lock the coupler to the tow vehicle, ensuring that it remains securely attached.
Remove a wheel is certainly an effective theft deterrent for unattended boat trailers