Originally Posted by MarkSF
The GPS will give you COG and SOG, but if you want speed through the water, you'll need a knotlog/paddle. If they are both nmea2000, you can use the plotter to display the speed from the paddle.
In fact if you buy a DST800, it'll give you speed and depth. Another $250 though.
.. and temperature, useful in crossing the Gulf stream etc...