You are absolutely right about water. I think few people are really aware of its tremendous heat capacity and its benefits either as a coolant or as steam.
If I remember correctly, liquid sodium needs exotic conductivity-based pumps and must be used in an alloy form. You are right, if you miss one detail it will freeze and will render the reactor and its associated equipment unusable.
The US Navy evaluated a sodium-cooled reactor in the 1960s but dropped the idea after these problems. The Soviets, on the other hand, went ahead and used this system in their Alfa-Class submarines. A couple times they experienced failures that ended up in the submarine having its reactor changes and scuttled in Novaya Zemlya. I understand it was one of the reasons why the Alfa-Class project was terminated. A sodium-cooled reactor and a titanium hull made these boats a very expensive failure.
Interestingly, many early reactors were graphite-moderated and designed to produce plutonium; electricity was a "by-product." The Chernobyl reactor was one of these "military" reactors, modified to produce electricity. One thing, however, that the designers did not take into account, was a safeguard system to prevent low power operation, and a backup cooling circuit.
The infamous reactors at Yongbyon in North Korea are also of the graphite-moderated type, hence the concern for its use as weapons-grade plutonium sources. What were the Soviets thinking when they supplied those reactors?
Be happy with what you have got