I played with Nokia Sports Tracker on my spiffy E71 today, generating this online report.
The start point was the ferry hard at Smuggler's Road, Bosham, a couple of hours before high tide (1700) in order to get back before dark (just after 1700).
So I was bashing up Bosham channel with the wind and tide, and bashing back down against both. The Sports Tracker graph of speed against time shows a speed of about 9kph up the channel, and about 7kph back again.
What I really like about the site is that as you run the cursor along the speed/time graph, a yellow dot runs along the satnav track to show exactly where you were. Very, very cool.