ehhhhhhhhhhhhhhhhhh?

We gotta remember what a lot of the Marine world really looks like, under the covers.

That is, lots of them will use HFO aka Residual Fuel oil or 'bunker fuel'.

Switching to Biodiesel? Probably the 'cheapest' of the options, not sure what if any implications exist from the switch (lots of ships will stop burning HFO in ports and switch to more common diesel/etc, however not sure if there is a difference in some engines with doing so long term)

Hydrogen Fuel cells are likely as much of a 'refit' from a labor standpoint as switching over to a nuclear reactor; Also the general issues of hydrogen embrittelment and the like have not yet been solved AFAIK especially for the volumes needed for large ships, also not sure if there have been a lot of studies as to whether the hydrogen embrittlement problem could lead to larger structural integrity issues on such a vessel.

Nuclear, OTOH, has had at least a few 'non-military' ships (mostly nuclear icebreakers) with good success.

The current 'whitewashing' strategy of cruise lines is LNG, for whatever -that- is worth...

Edit: finger slipped and hit post too early, so a bit was added, apologies!