Those statistics are easily available for US ports.[1][2] Ship tracking data is available as well.[3]

Counting containers in ports is more of a measure of port congestion than traffic. Also, a lot of them are empties, which you can't see from orbit.

[1] http://www.portoflosangeles.org/maritime/stats.asp [2] http://www.portofoakland.com/maritime/containerstats.aspx [3] http://www.marinetraffic.com/