Diesel doesn’t burn well.
Heavy Diesel doesn't, but light Diesel is not much different than kerosene/jet fuel. In fact the army uses JP-8 in everything, including diesel trucks, diesel HUMMVs, Bradleys, self propelled artillery. Aircraft crashes show it burns well enough, especialy when an aerosel.
That said, many ships don't use diesel, they use a much heavier fuel, those that do use diesel may use a heavier version, probably similar to that used for train engines, that indeed does not burn as well.
Still, it does burn.