I have one of these and have found the replacement hydrostatic release part
on the Internet. This is the same part that I have on my liferaft.

I believe that it's a mechanical part with a sharp blade that cuts the
rubber retaining rob in the EPIRB unit and (I think) a line for the life
raft. I tend to replace them every 2 years. They aren't cheap.

I've taken an old unit diving with me to see if it would "fire". It did
and there wasn't any explosive sound. I suspect that it's a pressure
activated release on a spring. The problem is that there's no way to
inspect it and it's a one time use device.

-- Geoff
www.GeoffSchultz.org

Geoff,
There is no rubber retainer in the EPIRB housing. The release module has
a pin that sticks through the front of the container that you can push
to deploy the EPIRB manually, or the water pressure releases the pin
when the boat sinks. When the pin is released it in turn releases a
long, flat spring along the bottom of the case that pushes the EPIRB out
of the case. I just didn't know what the module uses to release the pin,
and if that mechanism degrades over time. In either case I've advised
the owner to replace it, but at this time I believe he will keep it till
the end of the season.
Red