Can flash memory in smart phones be recovered after being under 10000 ft of ocean water?
Yes, but they will degrade over time.  The silicon die itself inside its protective plastic or epoxy package will be fine until the seawater eats away the pins and water actually gets access to the die itself.  Even then it may be possible depending on a number of factors.

The pins, however, might corrode in a way that stops further seawater ingress to the die.  If that happens they may last for years or decades.

I expect that, should they be recovered in a year, there will be some small percentage of mobile devices that, with careful techniques, will have recoverable data on them.