Picky, got any information on the pinger circuit location relative to the memory circuits? There would be a current field in the sea water if adjacent to memory. Current= accelerated corrosion. If in the same compartment, not good.
In any case, very high pressure seawater is likely to intrude into chips by way of the leads over time, although potting may delay intrusion. Circuit immersion for any long period of time is not good. Lets hope for success, but understand we may be too late.