In the grand scheme of things, it's not very much
1) that's still a shit ton of emissions.
2) just because there are other things that use more doesn't mean it's not a shit ton of emissions. Nor does it change the negative impacts of the high sulfur content in the bunker fuel (sulfur oxides), oils leaks in the ocean, or other harmful effects they have on the ocean.
“International shipping produces nearly one billion tons of CO2 emissions, which is approximately 2 to 3 per cent of total man-made emissions,” says Tristan Smith, a reader in energy and shipping at the UCL Energy Institute and leader of the UCL Energy Shipping Group. “This needs to reduce rapidly if we are to avoid the risks of dangerous climate change – at least halving in magnitude between now and 2050.”
"The emissions from 15 of these mega-ships match those from all the cars in the world."