The nuclear salt-water rocket is like the Halfbakery. Continuous Burn Nuclear Rocket.
The nuclear salt-water rocket uses nuclear fission isotopes dissolved in water. Normally, that would be instant death, as the high-energy neutrons emitted by the fuel would be moderated by the water, which makes
them much more likely to break another fuel nucleus, and chain-reaction hilarity ensues. To avoid this, the solution is stored in a mesh of boron carbide tubes, which absorb the neutrons, and stop things getting out of hand, until it is squirted out of the back of the rocket.
Maybe. It is a real idea. I will add a link. But you won't find me hitching a ride on one of those babies anytime soon.
Suppose the nuclear fuel was added to the water as powder or fine droplets. If the scale of the mixing is finer than the mean free path of the neutrons, the neutrons won't know if they are in solution or not. This is the equivalent of Low Explosive: all the bangy bits are there, but aren't mixed at the molecular or atomic scale. Provided you store the water and the nuclear fuel in different places within the rocket, all should be well without the boron carbide.
This should make the rocket efficient. The nuclear fuel is not a big part of the total mass, particularly with weapon-grade fuel. Most of the mass is in the water that is used for moderating the neutrons and as reaction mass to drive the ship forward. There is plenty of water in the outer planets and Oort Cloud (Last Water before Heliopause).