Actually, these boats (ships) use bunker fuel. It is a grade below diesel and is "refined" better onboard to be used as "diesel" for the engines. (I have been on a lot of cruises guys and learned a LOT about the ships. Also, I use to play the market on shipping companies. Wow was that interesting, but dangerous to play in the market!)
I wrestled with reality for years, Doctor, and I'm happy to state, I finally won out over it. (Jimmy Stewart in "Harvey" 1950)