This is only kind-of true. Ship engines are dirtier in the sense that they produce worse NOx/SOx (causes of acid rain) and particulate matter (causes of haze and breathing issues). These issues are serious, but they have led to countless articles with very misleading headlines like "Big polluters: One massive container ship equals 50 million cars"[1].
For global warming (CO2 and other greenhouse gases), shipping does create a decent fraction of emissions (~2% of manmade co2 emissions in 2018[2]), but nowhere near what land-based transport creates (trucks+passenger cars put out roughly ten times what boats do [3]). Attempts to address the particulate/sox/nox has included LNG powered ships[4], which allow unburnt methane to escape, which results in worse GHG emissions (but less of the "emissions" that allow headlines like "shipping is as bad as 50 million cars!"). And if you look at a different statistic, ocean shipping of goods seems almost responsible (eg, ocean shipping has the lowest "CO2 per tonne per km", far lower than rail/truck shipping[5])
All that said, reducing GHG emissions from shipping is great and necessary. I hope this project works out.
[1] https://newatlas.com/shipping-pollution/11526/
[2] https://maritimesky.com/2020/08/05/imo-has-published-its-4th...
[3] https://www.transportation.gov/sustainability/climate/transp...
[4] https://theicct.org/news/fourth-imo-ghg-study-finalreport-pr...
[5] https://timeforchange.org/co2-emissions-for-shipping-of-good...