[TorpX]

Originally Posted by Sailor Steve

Actually drag is worse on the surface due to wave resistance. As the hull moves through the water waves build up along the waterline, and the waves create more drag.
http://www.mecaflux.com/en/ship%20Hydrodynamics.htm

I don't believe this is true for WW II boats. Modern nuc subs are slowed by the bow wave on the surface, but most of the WW II boats were designed for surface travel. At least this is what I've read.

My first thought is the Schnorkel, and by using that you should technically be able to save lots of fuel for long voyages.

I doubt this would be the case. One of the results of the snorkel was that fewer boats were 'on station'; that is it took longer for them to go to and from their patrol areas. In other words it's only advantage was in avoiding aircraft attacks/detection, in every other way, it was a disadvantage.