I've owned two boats with Universal Atomic 4s, but engines were fantastic. The first had been sitting for 6 years with water lapping around the engine block for at least three years. After changing the oil, points, plugs, condenser, distributor cap, rotor, then leaning and adjusting the carburetor that engine started right up and ran like a fine-tuned Swiss watch for the next 6 years. The only thing I had to do was change the spark plugs every spring and be sure to treat the gasoline with Marvel Mystery Oil every time gas was added to the tank. This engine was raw-water cooled and eventually, a small inspection plate sprung a pinhole leak on the side of the block and the plate was replaced with no problems since.
My current engine is a Universal Atomic4, runs great, has been updated by Moyer Marine, fresh-water cooled, and provides me with 12 to 15 MPG fuel economy, depending upon the weather conditions. That translates to 600 to 750 miles of cruising range in a single tank of gas--not bad for a 37-year-old engine. Maintenance is about the same, change the oil regularly, change the plugs and filters once a year, and use Marvel Mystery Oil with the fuel. The Universal Atomic4 has a great reputation for longevity and durability--that's why they were so popular in military vehicles. Most WW-II vets I've talked with said you had to shoot the A4 in order to kill it, but it always took more than a single shot to do it in.
As for transporting diesel V/S gasoline in jerry cans on deck, I'm not too concerned. The cans will be tightly strapped down to the deck, similar to the way a marine battery is strapped down--not bungee corded to stanchions like so many that I've seen. Even in a knockdown situation those strapped down cans would still be in place.
I suspect that if you had a lot of fuel vapor in an enclosed area, gasoline, being far more volatile, would be easier to ignite with a small spark. Keep in mind, though that both gasoline and diesel fumes are only explosive when the correct amount of oxygen is available. One of the neat things I learned many years ago while being schooled at Exxon in Seattle, WA was gasoline vapor can be too rich to ignite. The instructor proved his point by tossing a lit match down the above ground vent of a 20,000-gallon, underground gasoline storage tank. The match immediately went out--the vapors were too rich to burn. He did the same trick with diesel. (I'm pretty sure there were students there that thought they were going to die that day.
Last summer I discovered something about jerry cans that scares the crap out of me, though. The feds, in their infinite wisdom, came out with new regulations for plastic jerry cans. They have been fitted with a new, leakproof spout that has a valvular ring attached to the outside. You must press the spring-loaded valve down in order for fuel to flow from the spout, which seems like a good thing. Unfortunately, there have been some major problems with the valves, some stick open, while others stay tightly closed--even when you try to open them. The black plastic that the spout assembly must be fairly tasty too. Squirrels, mice, chipmunks and other critters love to eat that black plastic and completely ignore the associated fumes. I watched a squirrel at our marina climb across a nearby boat's docklines, hopped on the red, plastic jerry can and proceeded to eat the top cap just like he was enjoying a peanut snack. The squirrel also ate part of the spout as well. With that in mind, I think I'll try to find some old, military style jerry cans for by trip to the south--just to be on the safe side.