This drawing compares the location of combustion chambers between many, if not all gas and diesel engines. This important difference becomes critical after an engine submerges or gets water, fuel, or coolant above the piston.
When an engine is hydraulically locked like this, there is a night and day difference between the two engine designs, and how to go about clearing the cylinder.
With the gas engine on the left, notice that simply waiting a while will allow most of the fluid to drain down through the piston ring end gaps, and eventually you will be able to rotate the engine. Or, to hasten the process, you can pull the spark plugs and turn the engine over.
Notice however, that the top of the diesel piston is constructed so that the fluid cannot escape unless the injector is removed and the engine barred over to clear the fluid.
(Some of this material excerpted from “PRACTICAL BOAT MECHANICS”, by Ben L. Evridge, to be published this fall.)