|It looks so simple...|
Diesel invented an engine that works by drawing air into a cylinder and compressing it until it is very hot (more than 1,000 degrees F). Just before the air is fully compressed, fuel is injected, creating a mini-explosion inside the cylinders. The expanding gases from the explosion push the pistons down, which are attached to the crankshaft by a connecting rod. The crankshaft, via the transmission, turns the propeller, and off you go. All it needs - besides water, impellers, filters, oil, belts, gaskets, etc. - is air and fuel.
Which leads me to the second part of this discussion - what's the difference between a gasoline engine and a diesel? Well, they are both internal combustion engines, which is kind of like saying they are small bombs encased in steel that we sit behind while driving. But they are small bombs in slightly different ways. A diesel engine compresses air and then adds fuel, which ignites in the incredibly heat. However, a gasoline engine compresses fuel and air and adds a spark for ignition. So while diesel engines are a delicate dance between air and fuel, gasoline engines have a third element - spark.
So why not use a gasoline engine on a boat? There are a number of reasons why diesel is preferable, the first one being that diesel fuel doesn't explode. That's a pretty nice feature when you're in the middle of the ocean and a giant fireball is the literally the last thing you want to see coming out of your fuel tank. Also, diesel engines are far more efficient than gasoline engines, and they last a heck of a lot longer, hence the reason you see so many diesel engines from the '70s still in use on boats today. Finally, diesels are a more simplified engine. They don't have as many electrical parts as their gasoline cousins, and electrical parts are highly susceptible to corrosion, a big consideration in a salt water environment.