Yes, it's entirely possible to have filled the entire crankcase with water/coolant, put the cap back on, cranked it, and "hydrauliced" it enough to bend or break a rod or piston.
Normally this happens when a head or cylinder cracks, or the gasket leaks, filling the cylinder with coolant after the engine stops and the valves close. Or when a float in a carburetor sticks, or an injector sticks, and fills the cylinder up with fuel. Either way, you cannot compress a liquid, so hydraulic force splits something, or bends something.
Remember that an engine displaces as much air (volume) BELOW the pistons as it does above. So air, or in this case water/coolant, has to move when it turns over.
I've seen gasoline get in the oil enough to make a vapor. Then someone tries to start the engine and the ignition lights off the vapor in the crank case and blows the valve covers and sometimes the oil pan clean off. It's impressive.