The reason propane is so popular is because it's a gas at room temperature but liquefies at a relatively low pressure (about 100-150 psi). That makes it easy to store (a thin metal pressure vessel lets you store it as a liquid), but burns as a gas so provides good heat density. Butane is similar but liquefies at an even lower pressure. As others have pointed out, propane and butane sink in air, so are a particular danger aboard boats as a leak will build up in the bilge. Fire is the main danger when that happens. Explosions require a rather specific ratio of fuel to air, and typically require it to be mixed rather than striated with fuel on the bottom, air on top. If the ratio is off, it will just burn (too much air) or simmer (too much fuel) rather than explode.

CNG (methane) remains a gas under pressure. So CNG tanks are typically designed to withstand 1000-5000 psi. They have to be much thicker - think scuba diving tank. This has weight and safety implications. The energy density for a given volume is also much worse than the liquid fuels. Also note that methane is normally odorless. The Gas Company adds the smell you know as the leaking gas smell, and farts smell because of sulfates mixed in with the methane. I would assume CNG sold as fuel also has the smell added.

The liquid fuels - alcohol and kerosene for this application - don't burn as well because they're liquid. You need to do something to increase their surface area to burn enough of it at once to really generate heat. Typically this is done with a wick, but it's not as good as a gaseous fuel. That's why people complain about long cooking times. Gasoline and diesel burned in an engine use either a carburetor or injector to vaporize the liquid to get around this problem. Pressurized alcohol stoves also gasify the alcohol prior to burning to get around this problem. But the lack of surface area (low rate at which it vaporizes) also has an upside - it makes the fuel safer.

Kerosene also has a problem in that it's not as clean burning as the other fuels. Like gasoline and diesel, it's actually composed of a lot of different hydrocarbons, each of which burns differently. So you're more likely to get other compounds than CO2, water, and soot after burning. That's why people complain about the smell.

Electric has a huge efficiency problem. All the fuels listed above are 100% efficient for heating applications. That is, all the energy in the fuel goes into heating up your cookware (before it dissipates into the air). Gas and diesel generators are only about 30% or 35% efficient at best. That is, 30%-35% of the energy in the fuel is converted into electricity. The rest goes into heating up the generator. So if you're using a 1000W electric stove, for every 1000W of heat that makes it to your cookware, about 2500W of heat is wasted heating up the seawater or air cooling your generator. So unless you're on shore power or getting all your electricity from wind/solar, this has huge implications for the amount of fuel burned for a given number of meals. If you do go electric though, I think induction is the way to go. It's more finicky, but it almost completely eliminates the risk of fire due to something flammable falling on the heating element.

Carbon monoxide is a higher energy state than carbon dioxide, so is typically a byproduct of insufficient oxygenation. The fuel wants to burn into CO2, but there's not enough oxygen so it becomes CO instead. As long as you have good ventilation to your burner (the outlets are clean with good airflow) and aren't trying to burn too large a quantity of fuel at once, you shouldn't have a carbon monoxide problem.

This tells you how much of a fuel you'll need to burn to cook identical meals. Lower numbers = more fuel needed; divide by 3.5 for gas electric, 3 for diesel electric. e.g. CNG stored at 5000 psi is 12.9 MJ/liter, so you'll need just about twice the volume of CNG to cook a meal as you would propane (25.3 MJ/liter). An electric stove running off a gas generator is equivalent to about 10.7 MJ/liter, so you'd burn 2.4 gallons of gasoline for every 1 gallon of propane used for cooking. Induction is probably better because it heats the cookware directly, though I'm not sure how much better.

I'm not knowledgeable enough about the day-to-day use of these fuels to comment on their relative safety. But understand that there are a lot of different factors which should go into your decision than just safety.