If we can operate submarines and aircraft carriers with nuclear, there is no reason why container ships and other cargo ships can't be nuclear powered. We could decarbonize the seas very effectively. It's not done simply because it is more profitable to use oil. 
I think portable nuclear generators are really bad in terms of wattage output, especially measured by weight. I might be wrong though. Their main draw is their ability to run for decades without needing to refuel. So, total energy to a charge is very good for portable nuclear but energy output per second isn't sufficient to move that much mass around quickly, or possibly even enough to beat the wind.
They don't share any details but I have to think that nuclear attack submarines have the ability to accelerate at some reasonable rate in order to be combat capable