Don't force the flywheel!
The engine is probably hydro locked (water above the pistons...and water (or any other liquid) does not compress.
If you're handy, pull the injectors or glowplugs, then rotate the flywheel. That will push any water out of the cylinders.
There is a reasonable chance that you may have one or more bent valves...or piston rods.
Bad news, but that's what happens when an engine ingests water...
Good luck...I do hope there is no damage to the engine.