Stephen,

in 1960 Jacques Piccard with 'Trieste' could withstand water pressure of 11000m under sea level!

http://www.sid-ss.net/write/trieste.htm

Well, he was not immersed into oil or potted in epoxy. The clue is to use the enclosure to shield electronics against high pressure. It would be the best to use a spherical shape.
I highly doubt, that electronics soldered on PCB and potted in epoxy will withstand this enormous pressure. It depends on elastic properties whether the potting material will show a volume shrinkage caused of applied pressure. But if this is the case, solder points and very sensible ceramic capacitors are exposed to extreme forces. I doubt, that this will not have any consequences...

Kai