Hydrolocking is a condition that happens when you get water inside your engine via the airbox. It usually happens in very deep water and can destroy an engine.
An internal combustion engine works by compressing an air/fuel mixture in a cylinder and igniting it. But air and fuel can be compressed, water cannot be compressed.
When water gets into the cylinder and the cylinder tries to move towards the valves to compress the air/fuel mixture for ignition it is stopped by the water. This can cause the piston arm to bend, it can blow the valves out, it can literally blow a hole in the engine block.
If you're going to be going through deep water you should know the basics of how to forge through water correctly, it will help prevent this kind of damage from ruining your trip.
