Primary production is mostly concentrated in the costal regions. The following map's colour bands are on a log scale: https://upload.wikimedia.org/wikipedia/commons/4/44/Seawifs_...

There's other reasons not to put PV in the middle of the oceans, like the storms, and that if you're willing to make suitable transmission lines of that kind of distance, you can also perfectly well connect the Nevada and Namib deserts and put panels in each so the time zones are powering each other after sunset/before sunrise).