True - I have also heard that diesel doesn't produce explosive fumes the way gas does. Forgot about that.
As for propane and dinghy gas... I imagine the engine compartment should be sealed against permeation by that sort of stuff. Proper stowage of tanks should ensure that. Still, it's a valid point and something to check for on an unfamiliar boat.
s/v Laelia - 1978 Pearson 365 ketch
s/v Essorant - 1972 Catalina 27