I have not played with the GPIO (yet), but based on the manual, you could connect a (fused) 12V source through a float switch to the GPIO In+ terminal and In- to ground. Then you would configure the Event Config / GPIO Trigger to Send Email.
I think it would email you a still picture, but if that's the only situation in which it does send a picture, you'd know that the float switch closed, even if the camera is looking elsewhere. If it's the regular bilge pump switch (you might want to put some voltage spike protection on the input because of the motor) you'd get an email every time the pump cycles. If you install a second dedicated switch just a little higher than the bilge pump switch, you'll get a notification email only if the water rises above the pump's cut-in; a sign of some problem.
Of course, if the problem is that the bilge pump is not running because the batteries are dead, the camera may not be running either, but that's a different issue (there are commercial vessel-monitoring products out there which would deal with that).
You're tempting me to do something similar when my winter cover comes down next weekend and I have to move the cameras inside. One thing - please let me know if you find a small weatherproof housing that fits these cameras. Most Pan/Tilt camera domes are huge (not to mention expensive).
1998 Jeanneau SO45.2