Yeah you're right flingwing, as a matter of fact I took my boat out for a sail this morning and made the silly mistake of turning down wind and down current at the same time and as you pointed out the boat maintained its speed relative to the water and the sails maintained their speed relative to the wind. If only the current and wind were exactly the same I guess the sails would still be on the deck ????
Ever think that the site refers to an observer in a position fixed in space separate to the system you are observing because its relevant !