Always moor you boat with a chain or cable. Use a good lock and secure the cable/chain around something that cannot be moved or that will not allow the cable/chain to be lifted over it or torn loose
Before leaving your boat moored, secure all removable items. Never leave fishing equipment unattended
Consider leaving your engine out of commission when you are away - remove the rotor, install a hidden cut-off switch, drain the fuel, remove a spark plug or the propeller
Never leave your keys aboard - even in a "hidden" place
Never overload your boat beyond recommended capacity
Secure outboard motors with special transom bolts or clamping screw locks