Hydrauliked means the cylinder pulls in water, usually from the leaking cooling system. Then when the engine rotates and the valves close it is not able to compress the water as it could the fuel mixture. Since the water will not compress, something has to give. Usually a connecting rod bends, a piston breaks or the cylinderwall cracks. If there is enough water in the cylinder to make it difficult for the exhaust valve to open, then a rocker arm could break. No telling what happened to your motor until you get the head off, unless you had water come out the sparkplug hole when you pulled the plug and turned the motor over.

It could also be a valve that is stuck, or even a defective rocker arm.

Pat