[acidburnNSA]

MSRs use either Th/U-FLiBe or U/Pu-NaCl and both of those would easily dissolve in water. But honestly radioactive inventory of these cores is small enough that one or two breached systems, while catastrophic, wouldn't have significant biological consequences as it disperses in the vast oceans. Certainly this wouldn't be acceptable for civilian operations but I wouldn't put it past an ambitious military. Problem with Molten Salt in a submarine is size. The fuel salt is very low density compared to metallic nuclear fuels, by about an order of magnitude. That would require more volume to be critical. Small thermal/epithermal solid fueled reactor cooled with water, liquid metal, or even solid fuel/salt cooled reactors (like the FHR) are appropriate for submarines but probably not full-on liquid-fueled MSRs.