Quote from procooling fourms-
"You can run them underwater....but not designed for, and therefore not suggested.
We can change case for this purpose, given an OEM order.
We test underwater, because we have a test tank we have been using for years.
The pumps often run months, during testing, hence is easier just to run them all underwater in a big tank.
We do these tests whenever we change supplier / materials / case design, etc.
Must be all those years at Boeing, got to test everything just to be sure"