Yeah, but one of the big bullet points is that the tube should be at negative air pressure in order to reduce air resistance.

Right, but what if its not feasible on land due to thermal expansion, but is feasible underwater with slightly higher power requirements to overcome air resistance? Just a thought.

The design is intended to be robust with regard to leaks (pumps can overcome some leakage to keep interior at a near-vacuum). This would not work underwater.

Pumps can overcome a small amount of leakage - from tiny cracks between bolt threads, etc - places that an un-reactive gas can make it through a huge pressure differential. In shallow water, one has the viscosity of water to work with, which makes this much easier in liquid phase.

What I don't know is how pumps, and cracks, would react to gaseous H2O versus gaseous N2; Or if perhaps a liquid with low offgassing potential could be used as a boundary layer to seal cracks instead of water (which, while much higher viscosity than a gas, remains lower than most liquids).

One big advantage of any submersed liquid approach: Earthquake resistance.

Additional notes: My main theory before the announcement was that the concept used a fast-speed-of-sound, low-density STP hydrogen tube and maglev rather than slow-speed-of-sound air tube, or an expensive-to-seal vacuum tube. An STP hydrogen tube hovercraft levitation model still seems pretty damn attractive to me, especially with a sealing liquid buffer of some sort - it cancels out the problem of water-vacuum interface, and in water, flammable seals, as well.