The idea of a 'U-plane' is a heavier-than-water vessel, kept travelling underwater controllably by 'wings', i.e. smallish fins analogous to airplane wings, but much smaller because the lift of water is much greater. It's a similar idea to the popular view of a shark, which has to keep moving in order not to sink. Such a thing could be much thicker than present submarines, and go to greater depths. Heinz Lipschutz liked to think the was the inventor of the idea, though I'd guess it was investigated without his knowledge. On the face of it, it would be an ideal application for a nuclear-powered motor.
Lipschutz's design included emergency flotation (by electrolysis) in the event of (I'm guessing) the propeller(s) being obstructed.
Incidentally this is NOT a dual-purpose vehicle able to travel in both the air and sea, as the name might imply.
(I thought I'd mentioned this in the section on submarines, but I find I hadn't).