In alt.home.repair, on Tue, 19 Jan 2021 19:15:02 +0000, fullofcupcakes
wrote:

I think the preheated water has cooled by now so it has contracted leaving a vacuum to suck the glass away from the cast iron. That should protect the water.

That works okay, but it's best to freeze the water in the water heater
so it doesn't jostle around when transported. You can take it to a
wholesale meat market or a similar place and they'll freeze it for you.