Shipping also uses RFO [1][2] (Residual Fuel Oil, often just referred to as bunker fuel). It is the absolute filthiest petrosludge that's essentially unfit for any other purpose, and the exhaust waste is dumped into the ocean directly. That 2.5% is likely a gross underestimate of the true impact [3][4]. The "scrubber" systems which are supposed to reduce the sulfur dumped into the air, just wind up dumping it into the sea [5].

  1: https://williamliggett.com/2018/05/28/bunker-fuel-sounds-bad-its-worse-than-it-sounds/
  2: https://wolfstreet.com/2018/06/03/bunker-the-fuel-for-the-giant-engines-in-large-cargo-ships/
  3: https://acp.copernicus.org/articles/20/11399/2020/
  4: https://www.sciencedirect.com/science/article/abs/pii/S026974912101280X
  5: https://www.frontiersin.org/articles/10.3389/fmars.2018.00139/full