Nuclear heats up water to turn steam turbines to generate power. There's still that loss from heat source to electricity stage they have to figure out.

Source: https://www.nrc.gov/reading-rm/basic-ref/students/science-101/how-does-nuclear-power-plant-make-electricity

The molten salt acts as both a heat storage buffer, and a backup shell if things go wrong. But if there's a catastrophic failure and seawater gets in, the salt will dissolve in water and spew potentially radioactive particles far and wide. Not sure this is a good idea for use in water. At least, on land the molten salt will stay put.