I have two systems on my boat.
I have a Dakota Alert DCMA 2500 ($120). This is a motion detector located in the cockpit
which wirelessly sends a signal to the controller. The controller is 12vDC so hooked straight up to my panel inside the boat. When someone enters the cockpit
and a signal is sent to the base station, it chimes and activates a 12v relay which I have hooked up to a voice module. The voice module not-so-politely asks people to leave the boat. If the person ignores this loud alert and forces opens the cabin
door the second alarm kicks in.
You don't really need the first system.
The second system is a basic Clifford car alarm (all of which are 12v). This is hooked up to a door switch and if activated, wakes the whole neighborhood up with alarms and flashing lights fitted via a relay to the boat lights and sirens. These systems are all DIY installations and fairly straightforward if you have a basic understanding of 12v systems. As this alarm is not reliant on movement, I have never had a false alarm (except for coming home drunk and forgetting to deactivate). Most simple car alarms can be hooked up to the press-button door switches that you can salvage
from any scrap car. A $5 relay can then be hooked up to the output alarm signal if you want to hook the alarm up to higher amp systems like your boats horns and lights.
In case anyone thinks they now know my security system, I also have a remote camera
which sends video and captures images
. A little bit OTT perhaps, but I am equipping my boat for a circumnavigation
and the goal is to simply make my boat less desirable to thieves than the next boat around the bend.