If you've sucked in water while it was running, pulled the plugs and you still can't turn it over using the flywheel and a pry bar more than likely you busted the crank or bent rods. Even with a busted crank you still should be able to get some movement if only a little.
As you've realized by now, water be non-compressable and sucked into an engine turning over at speed, is going to do some major damage.
Busted crank, bent rods, blown head to name a few......Mike
**EDIT**
Do a search on the net for hydrolocked engine. Not to be mr. doom and gloom but here's a link I picked up on another site, look at the con rod
http://img38.exs.cx/img38/833/dcp021217fu.jpg