^What are you going to use for fuel in your fuel cells?
Whatever amount of energy-per-time it takes to move a loaded cargo ship through the water with conventional fuels, it will take the same amount of energy-per-time with fuel cells.
I had read somewhere that someone's planning (or has already done it) to use high-flying kites to assist in moving ships along. Now that's (more or less) "free" energy-per-time. Of course, that's just sailing with unconventional sails.
Terry
NOTE: There are salt water batteries, and in fact that was one way to power military torpedoes. But there's still consumption of raw materials... which took energy to refine and process. Example: some navigational buoys are run with salt water batteries, where the anodes are discarded Volkswagen magnesium engine blocks. Imagine the energy investment in mining, transporting, refining, and fashioning those engine blocks.
TANSTAAFL
"There Ain't No Such Thing As A Free Lunch." (Heinlein, I believe.)
That's one of the biggest pitfalls in bright ideas to save energy: the fact that the infra-processing takes energy, too, although that can sometimes be amortized out. But let's face it, even the headlights in your car don't light up the road for free.