You always order speed agains water (air) .. not agains ground. If you order stop, you will drift. If you order speed of current in oposite direction of the current, you will stop.
How does sub measure speed IRL I don't know. They can't use GPS (which could give speed agains ground). The most probably have some inercial position (thus speed) system, but how precise that can be ? Who knows ..
Aircrafts uses mainly speed against air, but hey can easily read speed against ground via GPS (and they do), same for surface ships.
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