If the engine was hydro-locked (water in the cylinder), pulling the compression release would have allowed it to turn over. If that didn't doit, there's some other condition at work.
Unless the engine slipped time during the crash (which is possible if the timing chain is in bad condition), there is no reason to think the valves would be bent, so I would not go ripping the head off until you figure out what's up. If you can't turn the engine over (gently) with the compression release pulled, remove the cam cover and check the timing. Correct it if it's off, and replace the chain, then go from there. If the valves are timed right, they won't suffer any further damage from cranking the engine even if they're bent, but if they are, there won't be any compression.
If it is or was just water in the cylinder, you should be able to clear it by cranking the engine with the compression release pulled, or if that's been removed, with the spark plug out of the head. If the whole bike was under water, it probably entered through the exhaust or intake (the carb will have water in it,BTW. You need to address that). If you get it cleared and the engine turns over in a reasonably normal manner, try the plug back in it and see if it has compression. If so, you have a decision to make: if the water was dirty, there is dirt in the top end. You can get away with that if the water was fairly clear, but you're taking a chance if it was muddy. Might want to go ahead and remove the top end just to clean it up at that point.