Many types of switch can be used. These are just some ideas.
Pressure mat intruder detection
Mooring rope security
Bilge water level & pump activation count
Reed switch intruder detection
Temperature sensor
OR gas alarm warning
Siren
Function relays enable activation of on-board systems from your mobile. The blue and green phone plugs seen here can activate bilge pumps etc. A simple system, it is easy to service or upgrade to your needs.
