Nuclear reactors are arguably a lot safer (in terms of meltdown risk) on the ocean than they already are on land, seeing as how they're in ships that float on a giant planet-wide heatsink that also happens to be exceptionally good at confining radiation.

The more pertinent risk would be piracy and the like; if thorium ever catches on, it'd probably help make that less of an issue.

We already have nuclear submarines and aircraft carriers. The popular joke is that all military/navy/etc equipment is built by the lowest bidder.

On the bright side: if a nuclear reactor melts down in the middle of the ocean, there’s basically nothing around to be inconvenienced by that event.

Those nuclear aircraft carriers have a bit more military protection than an arbitrary nuclear cargo ship will.

And a sliiightly bigger budget, both construction and maintenance.