Originally Posted by OS2Dude
GPS (Ground Speed) is MUCH more useful. It is your ACTUAL speed. The water speed is how fast the water is under your hull. If you are at anchor and there is a current, you will display a water speed but not a ground speed. If you sail WITH the current, water speed will be less (possibly even negative) than your ground speed. If you sail against the current, you can show a much greater 'speed' than what you are actually going compared to ground.
Don't know where you sail but having both SOG (Ground Speed) and
STW (speed through water) can give me important information on the direction of the current.
When I sail with the current I will try to use it to my advantage.
When the current is against i seek into water with less depth as I know the effect is less there. Short tacking in water with less depth is better if the current is against..