Nearly 40% of shipping volume worldwide is moving fossil fuels (coal, oil, gaz). The easiest way to reduce drastically shipping emission is reducing usage of fossil fuels...
https://www.ics-shipping.org/resource/shippings-role-in-the-...
https://www.ics-shipping.org/resource/shippings-role-in-the-...
We're talking about whether the shipping industry will choose to improve how their ships operate, but you're talking about new powerplants on land would remove the need to even have those/as-many ships at all.