I would be tempted to use a gps module as these are accurate to 10m and your project, seemingly, would be comfortably serviced by that level of accuracy. Once the alarm gets triggered, it can be reset so it does not trigger again until you have gone out of range. The gps modules available allows uart etc interfaces and would be a lot cheaper than boat repairs. Include a button to register 'home' in case you move around. These modules also, typically, provide speed and altitude, the latter being useless for your project. I don't know whether Microchip have libraries for their micro-processors, but Arduino does. My personal preference would be an ESP32 processor that also provides wifi, which could be accessed from your mobile phone/device, which might be useful for other purposes.
Most important is that the gps antennas are weather proof and can have quite long interface cables, so the micro can be inside the cabin.