If you look around the site a little bit, you find that this is actually just a maritime-oriented rendering of data from OpenStreetMap. Not as usable as professional, commercial maps, but still not too bad.
Hopefully this encourages more mapping of features not on land for OpenStreetMap, just as OpenCycleMap has helped the availability of cycling data.
Hopefully this encourages more mapping of features not on land for OpenStreetMap, just as OpenCycleMap has helped the availability of cycling data.