How do you know if you cracked your engine block

You will have to do a compression test on each cylinder and if the cylinders don't hold the compression then ya got issues.... your local auto parts store may have gauges to do this or bring it to a shop.

Sounds like you hydro locked your engine. could be a bent/broken valve, hole in the piston, cracked wrist pin, broken connecting rod, cracked head, etc. Pretty much think of anything you would need to disassemble the engine for and there ya go. When water gets in the intake and sucked into the cylinder, its not a good thing. Water doesn't compress like air does. Something's gotta give. The water isn't, so something in the engine does. Pull the plug and drain the oil. Check for metal. If you don't see any right away, drop the pan and see if there's anything in there. Then pull the valve covers and see if there's any bent or dropped valves. Either way, your looking at total disassembly to fix. Could also be cracked cylinders, but the engine wouldn't run at all if that were the case.