Nuclear reactors can work just fine with other coolants, and really high efficiency nuclear reactors here on Earth are designed to use Helium in the primary coolant loop in order to support running the core at a much higher temperature than water cooled reactors generally run at. Other work arounds have included using Sodium (very dangerous because Sodium will ignite in the presence of water, so a leak could become a flaming radioactive nightmare) and metals like lead (the opposite problem has occurred, apparently some Soviet era submarines using high powered, lead cooled reactors have been decommissioned because the operating temperature was allowed to drop too much, resulting in the coolant solidifying in the primary loop).
The biggest reason to avoid "steam" powered generators isn't technical, but rather the laws of physics. Steam generators are considered to be "Rankin" cycle machines, and like most heat engines have a pretty hard upper limit to the amount of energy that can be extracted, known as Carnot's theorem (thermodynamics)
Carnot's theorem states:
All heat engines between two heat reservoirs are less efficient than a Carnot heat engine operating between the same reservoirs.
Every Carnot heat engine between a pair of heat reservoirs is equally efficient, regardless of the working substance employed or the operation details.
The formula for this maximum efficiency is:
efficiency = 1 − TL/TH
where TC is the absolute temperature of the cold reservoir, TH is the absolute temperature of the hot reservoir, and the efficiency is the ratio of the work done by the engine to the heat drawn out of the hot reservoir.
For steam engines (which is what we are talking about), 33% is a high efficiency without reheat or other additional steps (combined cycle systems get much higher efficiency because they use the energy in the fuel several times, i.e. a gas turbine generator which uses the exhaust to heat the steam).
One thing which might work well in space is to use an MHD generator, where the heat of the reactor ionizes a coolant that is them passed through a magnetic field. MHD is not constrained by the Carnot limit. The ultimate expression of that is a "fission fragment" reactor, where the fissile material is introduced into a magnetic chamber in the form of fine dust. The resulting fissioning of the material is captured in the high energy movement of the fission particles (up to .03 c, but in practice usually .01 c due to internal collisions), making a high energy stream of charged particles to be tapped for electrical energy or to be used as a rocket engine. (See this as well)
MHD and fission fragment reactors also require less "plumbing" and usually smaller radiators for the amount of reactor power, which are all advantageous when designing a spaceship or space colony.