Didn’t the Soviets play with Liquid-Sodium reactors on a sub or 2? I don’t think that they had a lot of success with it.
So did we. USS SEAWOLF (SSN-575) had a liquid sodium moderated reactor at launch, but it never did work right, and later it was ripped out and replaced with the standard naval PWR (pressurized water reactor). This was back in the late ‘50s.
The Russian Alfa-class subs of the 1980s used molten lead for coolant!