My advice would be to stick with the propane. Yes, it is heavier than air and therefore you need to be mindful of certain precautions when using it, but the other fuels you mentioned also have their own issues. Alcohol stoves require a fair bit of attention too, and you have to treat the fuel storage much like you would gasoline. Kerosene/diesel stoves burner elements require pre-heating (usually with alcohol), which can be a hassle, and over time tend to leave a residue/aroma on the boat's interior. If you or any of your crew has a sensitive nose (as one of my kids does), this could be a nauseating problem.
Your boat is already set-up for propane. If you maintain the system properly and take basic precautions, it should provide reliable service. If you don't have a remote solenoid shut-off for the propane system, be sure to install one or have it installed if you're uncomfortable doing that sort of thing. Use the solenoid religiously, like an on-off switch for your stove. If your propane stove is cantankerous, definitely rebuild it or replace it with a new one that has all the safety features (like thrermo-couples).
If you can't sleep at night because you are really worried about propane saftey issues, many propane stoves can be converted to CNG, which is lighter than air and therefor safer. But be advised that CNG can be hard to come by. I have heard that there are outfits that will ship it to you. This might work if you're not out cruising and don't use a whole lot of fuel.