>Reason is ofcourse that the US military has made it inaccurate for commercial purposes

Selective Availability has been turned off for the past couple years, greatly improving the accuracy of civilian receivers and reducing the need for differential correction for most purposes.

>Sea and Ocean current nearly always offset the speed reading

When used for navigation, I'm not really concerned with how fast I'm going relative to the water. I want to know when I'm going to get to my destination. (If I'm surfing a standing wave in my kayak, my speed relative to the water's surface may be very high as I plane down its face, but I'm going nowhere downriver.)

We do have to consider current, however, if we're attempting a speed run from point A to point B. Certainly, if you have a favorable tide, or are riding the Gulf Stream, you will get there faster. (Just ask the sailors that make the Newport to Bermuda run.) For this reason, you would not want to try and make any claims about a boat's top speed based on runs that were made in conditions that involved ocean current.