There is a 3 to 1 reduction in the gearing to the props and as a practical matter the engine RPM in water never goes above about 3400 RPM so the props normally never run above about 1100 RPM. Somewhere I seem to remember Hugh saying something like "be sure to disengage props when out of the water since the RPM can easily exceed the maximum prop RPM. I was a bit curious about that remark though since on the road the engine RPM rarely would go much above 5000 RPM which would give you a prop RPM of about 1650, not all that much higher than in the water. One thing I've noticed though is that the prop shafts run quieter when there is a water load on the props. This is probably because those prop shaft u-joints really flex a lot and the load keeps them from chattering. I'd say generally it's a good idea to disengage the props when out of the water, especially if your driving at much above "parade" speed.
I have never worryed much about spinning props out of water, as John says they spin much slower, no I would not do 60mph down highway with them on
You can hear them engaged and makes clutch work harder,
I ususally turn them off when shifting after coming out of water but sometimes forget and Amphis behind panic tooting horns etc.Leaving turn signal on is a more important thing to toot at.
but do remember you must be totally stopped before you engage them going into water