Hydrolock
In automotive terminology, a hydrolock (short for hydraulic lock) is the immobilization of an engine's pistons by a liquid (usually water, hence the prefix "hydro-"). Hydrolocking occurs when liquid fills a cylinder on the intake stroke and, due to the incompressibility of a liquid, makes the compression stroke impossible. This, in turn, prevents the entire engine from turning, and can cause significant engine damage if one attempts to forcibly turn over or start the engine. Typically, connecting rods will be bent, making the engine uneconomical to repair.
Causes
It is relatively common when driving through floods, either where the water is above the level of the air intake or the vehicle's speed is excessive, creating a tall bow wave.
Another reason for it to occur is in the event of the head gasket cracking or "blowing", which causes the radiator coolant to mix inside the combustion chamber.
Treatment
Regardless of the cause, treatment is the same. Removal of the spark plugs will allow access to the combustion chamber. By inserting an irrigation syringe fitted with a flexible plastic tube on the end, you can suck out the liquid that infiltrated the combustion chamber. Once all the fluid is removed, the chamber should be sprayed with a liberal coating of a water displacing lubricant such as WD-40, and allowed to sit until the remaining water has evaporated. Once the water or coolant has been removed, inspect the combustion chamber for rust. The best option would be to remove the head, but a borescope or small flashlight and mirror will also work. Light surface rust will be normal, but if you see any heavy corrosion, it should be gently removed, and vacuumed out of the chamber. After inspection, pour a tablespoon of assembly lube (such as Marvel Mystery Oil) or clean engine oil into each combustion chamber. Attach a socket to the crankshaft, and slowly rotate the engine through a few revolutions. (Make sure that you turn the engine in the proper direction, see a service manual for proper rotation direction.) If you hear any clanking, or harsh scraping, you should bring the engine to a competent mechanic. After verifying that the remaining systems are free of water (electrical, fuel delivery, air intake, distributor, etc.), re-install the spark plugs and wires, and attempt to start the engine.
Steam Engines
A hydraulic lock can also occur in the cylinder of a steam locomotive and is both damaging and dangerous, because a broken connecting rod can puncture the firebox or cause a derailment. Steam locomotive cylinders are always fitted with cylinder drain cocks which can be opened by the driver to release condensed steam.
SeaFoam
Some people have stated they think Seafoam can Hydro lock their motors. Used correctly this cannot happen. When drawn through an intake vacuum hose into the intake, and dispersed with air amongst all the cylinders, there will not be enough liquid in any one cylinder to cause enough incompressibility. For example, a Ford 4.6 liter V8 can hold (up to) 575 milliliters of fluid in each cylinder. 1/3 of a 16 ounce bottle of Seafoam is 158 milliliters.