Yup. They burn heavy bunker fuel - the sludge that is too bad to be used for anything else.
Considering the amount of shipping, it’s horrendous.
But - and there’s always another view - I don’t know how much energy you’d need to use to haul that much cargo by other means like rail and trucks. One container ship carries as much as a thousand trains could carry. Vessels are really, really large, which make them quite effective.