There's also the potential for nuclear powered vessels to act as tugs. Hauling a barge gives them increased capacity but they could effectively double/triple the capacity of a barge by towing a cargo ship instead.
Not saying it would be a common practice but if you're in a relatively safe area it's an alternative to trying to find fuel for all the diesel ships, particularly if you need cargo moved now instead of later.
Edit: There's a decent list of nuclear powered surface ships on the following link (it's just a matter of figuring which ships survived!)
http://www.radiationworks.com/nuclearships.htm