Ken's right that commercial or wide-area satellites aren't the complete answer, but in conjunction with the Automatic Identification System they are a big contributor to a common operational picture.
By the way, there's been serious discussion of a geostationary imaging satellite with up to 10 m resolution:
http://www.congrexprojects.com/custo..._GeoOculus.pdf
Not so long ago - even in the 1990s - the oceans were a big black hole with a few moving spotlights of airborne radar and the occasional swath of a radar sat (of which there were very few). No longer.