I spoke with the folks at Garmin, and the following is an update concerning GPS compatibility with PLB's (using the NMEA 0183 optical standard). Also,
charlie s charlie was correct about their waterproof standards.
ALL Garmin products meet the same standards (including the eTrex).
Most Garmin GPS units are waterproof in accordance with IEC 529 IPX7. IEC 529 is a European system of test specification standards for classifying the degrees of protection provided by the enclosures of electrical equipment. An IPX7 designation means the GPS case can withstand accidental immersion in one meter of water for up to 30 minutes. An IPX8 designation is for continuous underwater use.
That said, the technical representative from Garmin indicated that if there is an active current, running water, or if you are moving a GPS through water yourself (i.e. swimming across lake or stream), then water
WILL get into the GPS and damage the unit in short order. So precautions must be taken.
Concerning the ability to upload coordinate information to a PLB (or other electronic device),
ALL of their current products follow the NMEA 0183 optical standard. The older products that are discontinued do not have this feature; however, their newer products (including the eTrex) are capable of this data exchange.
If I decide to put a small GPS unit in the vest, I'll probably put it in the following waterproof bag...
It's called the "Voyageur Squawk-About Two-Way Radio Case" and was designed for cell phones. The neat thing about this submersible bag is that it has a cable connector built in, which keeps this a closed system. Therefore, all I would need to do is solder a male plug onto the PLB optical cable (the ends of the wires are bare and requires soldering anyway) and connect that to the plug on the exterior of this case. For the inside of the case, I would solder a Garmin data cable (shown below) with the appropriate connector.
It'll be a while before I receive all the basic components. Then, I can make a few more decisions on the final configuration of the vest (including wether or not to add GPS or pyrotechniques). When I finished, I'll upload some pictures of the final product.