I believe the answer is no.

Even modern subs will patrol with a knot or two to keep some depth control.

Normally boats would trim to slightly bouyant and then use the planes on a slight down angle to keep them down - it gives better depth control, and the boat will naturally rise to the surface if the propulsion fails.

Exact trimming would be very difficult, and require fine trimming tanks and pumps that weren't in the designs of a fighting boat.

Occasionally US boats would find a thermocline that was strong enough to 'sit' on - the boat would actually rest on the denser layer of colder water, staying submerged at a constant depth with no forward motion (and no noise), but this is not really neutral bouyancy, but rather resting on a (fluid) surface. The thermocline was in general too deep (600 feet or more) in the Atlantic for U-boats to do this trick - at least I don't recall any such incidents.