Why aren’t we using solar powered ships?

I think the larger the craft, the more this would make sense. I had the idea of a long pontoon craft for inter waterway trade powered by solar panels based on a patent I had seen[1]

Although that was for a ground-effect craft[2], the long flat deck seemed perfect to mount solar panels. I'm not well versed in hydrodynamics, but I imagine the smaller contact area with the water would reduce drag for the initial launch phase as well.

[1] https://www.google.com/patents/US5746146

[2] https://en.wikipedia.org/wiki/Ground_effect_vehicle

I too have wondered this. I think these guys have done some research on the matter: http://www.seasteading.org/

...but I don't know of the current state, if any, of their conclusions.