Simon, it might pay to visit an electronics tech who is also an authorized
Raymarine shop. What you can do will depend on how
Raymarine is powering their alarm, if it is a small piezo chip and they are using a limited-power driver to make it chirp, they may be able to "simply" insert a driver transistor or relay to drive a higher power louder buzzer or alarm circuit. But if things aren't so simple or robust--just hacking in could blow up the whole thing. Better to pay someone who can tell exactly what their driver is up to.