I can't speak to undersea cables, but back-to-back AC/DC inter-connections certainly are used between grid sections, as they allow for easier control of both frequency and phase. One languishing project called Tres Amigas originally intended to tie all three US power grids together in northern Texas, though, as with many large power infrastructure projects, that one suffered from the bureaucracy and conflicting goals of too many stakeholders.

There are a few high voltage DC transmission lines, and more planned or proposed, which allow for much greater control of power flow in a network. Places with high penetration of renewables and relatively low demand (like Iowa, for instance) could see value in more directly exporting wind power, delivered to the point of load (say, a population center like Chicago). Designing AC transmission for such a project would be more complicated due to the way power flows around AC grids.

Yes, submarine power cables quickly have to be DC due to capacitance between the cable and the ocean. Compensation for that is neither easy (middle of the ocean) nor cheap.