I am a firm believer that the best product installs on the boat with dedicated GPS sensor, detects the sudden absence of a fob, raises an audible alarm, closes a contact for additional alarms to be added (or close the MOB contact on a B&G H5000 or similar), opens a contact for disabling things (e.g. like an engine), has a contact for an acknowledge/silence button (ends alarm, resets contacts) to be installed anywhere on the boat, and transmits an nmea sentence to an MFD to aid in navigation back to the MOB. The acknowledge/silence button would not remove the MOB from the MFD; such removal is done at the MFD as a separate step.

I am a firm believe the fob is created with a speaker to alert the user the boat is out of range (so the user can return the fob back to the boat owner when this happens at the dock), a blinking light to show it is functioning, chirps when it is turned on and chirps when it turns off, some foolproof way to insure the device is OFF when not in use and ON when in use, such as a receptacle the fobs can sit in while not in use that can trigger a magnetic switch in the fob to turn it off. My preference is the fob has a user-replaceable battery; waterproofing is marginal, so costs can be kept low, and has simple user instructions to check for blinking light and return to the boat immediately if alarms. 

To keep costs way down have expectations that an MFD is integrated, rather than a display for lat/long, and do without features to summon help off the boat. The ideal  product would solely excel at the purpose of helping people aboard the boat to immediately respond and rescue a person or pet that is MOB and excel at having inexpensive key fobs so many can be purchased and/or spare.

IMHO - Dan

Dan - Yes exactly this would be absolutely ideal.  Just add that MOB tags should come in variety of sizes/formats.   The best solution is one that people will use consistently.