A PLB needs a clear view of the sky. I don't think they work if submerged or through obstructions (the upside down hull). I know some marine 406MHz beacons are intended to signal if the boat sinks but I assume they're carefully mounted so they will float free of the boat and self-right so the antenna faces up (or have several antennae).
With many EPIRB's (marine precursor to the PLB) it is required that the device be allowed to float in the water to work properly. They use the conductive salt water ocean surface as a ground plane for the antenna.
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