Zooming in around Calais, for instance, you can see ferries making way and their wake, bearing in Google Earth are correct, and the coordinates are very correct, in fact better than some Admiralty printed charts that I have. I wanted one day to check the alignment of Dover and Calais together, and was surprised of the quality of Google Earth's references. There are no soundings of course, no tidal data, identifying buoys is impossible unless there are pictures, etc, but details and mapping are quite good.
To find channels, have a look at AIS data .... why not? Map details can sometimes seem strange in SSEx, but the topographic data are excellent.
Regards,
Luc