While I'm slightly bullish on MSR's in general, I think a MSR on a ship is not a good idea. A thin-skinned reactor vessel (one of the big advantages of MSR's as they operate at atmospheric pressure) with a fuel salt that is(?) water-soluble sounds like a disaster in case of a ship sinking.

At least with a traditional water-cooled reactor there's very thick walls for the seawater to corrode through, and then the fuel itself is in a relatively inert oxide form. Similarly for a lead-cooled reactor, when the ship sinks the lead would solidify creating a big radiation protection armor around the fuel elements.