Navman 8084 Owner's Manual page 4

Water speed and ground speed
A paddlewheel sensor and a pitot sensor
measure water speed, the boat speed
through the water. A GPS measures ground
speed, the boat speed over the bottom of
the water. If there is a current, then these
speeds will be different, and the log, trip
log, economy and range will be different, as
shown below.
4
Water speed is better for measuring the
boat's potential performance, Ground speed
is better for going to a destination because it
takes currents into account.
NAVMAN 8120/8084 Fuel, AIS, & Radar Addendum