What is the difference between speed over ground & speed through water ?
Speed over ground is the speed of the ship with respect to the ground or any other fixed object such as fixed buoy or island.
Speed through water is the speed of the ship with respect to the water such as anything floating on water.
A ship with her engine stopped in water with 2 knots currents will have zero speed through water but will have 2 knots speed over ground.
You can read in detail on this blog post