With a tidal stream running, a GPS ‘Man Overboard’ position is of little practical use unless you’ve noted the time the button was activated. The Fix is, of course, a geographic one, and the casualty is immediately drifting away from it. After six minutes, a two-knot stream will have taken the victim 400 yards from where the GPS says he is. After fifteen minutes, he or she will be far out of sight even on a calm day.
This doesn’t affect your own ‘eyeball’ efforts to recover your crew, because both the victim and the yacht are subject to the same set, but if you call for the helicopter and expect the casualty to be where your GPS says he is, he won’t be. The SAR people can work out the set and drift using the MOB fix as a starting point, but they will need to know how long the casualty has been in the water.