You didn't say how much water, or for how long. And how the water go into the engine (through the carb, or the car was stuck under water in a creek for a month?).
If the engine was full of oil when this happened, chances are a piston ring(s) is rusted to the cylinder wall. First I'd get oil back into the engine. You don't want any moisture to get down on the rod or main bearings so protect them with fresh oil. Then you might try pouring some penetrating oil (Kroil brand is supposed to be really good) into the spark plug holes and let it soak for a while, hours or days. This is experimental, depends on your patience. Then put a socket or wrench on the rear lower engine pulley and try moving it back and forth a bit. If you can get it completely free, change the oil again and run the engine. See how lucky you are, does it smoke like a banshee, or purr like a cat?
If it's still frozen then the next step would be to pull off the cylinder heads and cylinders, one side at a time. If that frees it up, then you either fix the problem. Hone the cylinders and re-ring the pistons, or replace them. If it's still stuck, then the problem is in the bottom end of the engine, the rod or main bearings.
Of course, all this assumes that it's not your starter gear that's stuck in the on position and locking up the flywheel.