It depends on your desire for realism. In real life, U-boats could not hover - without movement of water over the dive planes caused by motion of the boat, they would either sink or rise to the surface. The same goes for any submarines of the period.
Later boats had much more complex automated trim systems that allowed hovering, but this wasn't the case at any time near WWII. The only exception was when a boat found itself able to park on top of a thermal layer - this was a very rare thing.
Hover all you like, but please don't pretend it's historically accurate to be able to do that. If you want accuracy, maintain at least 2-3kts when submerged, especially when near the surface. The faster you go, the more chance the planesmen have of keeping you at your required depth.
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