You are correct, I'd forgotten the other 3 mainly as they were all failures. Commercially there were 4 nuclear powered ships built. None of them was a commercial success due to a number of reasons not least cost. The one still working can hardly be classed as commercial as it's owned by a branch of the Russian government and being operated by their MOD in the Russian Arctic. She is also an ice class vessel. She cost US 265 million in 1988. A modern large tanker (310,000 tonnes) costs about 92 million today. All other nuclear powered ships are military so far as I know. Currently in operation there are about 53,000 merchant ships in use and in 2017 they moved 10.7 Billion tonnes. You'll find that most diesel electric powered ships don't have conventional propellers but instead use azipods and thrusters. As I said apart from cruise ships they are mainly found on small vessels like supply boats.