Slitherine

Pocus, in the new stream, mentioned naval support for land units. Is this new to the DLC or is it now in the basic game? If so, how does it work? I have always been land-based!

Yes that's a novelty of the DLC, the expansion adds 'regional decisions', that you might know about if you played AGEOD games with the old engine. You can with this decision target a region which has an enemy army, provided it is both adjacent to one of your army and one of your fleet (in the same region as your army works too, so it would work against a besieged enemy or on the contrary to fire against a besieger). This will inflict effectiveness and health damages.

Does it count if you army is in the water as navy ships? Thinking about how to use this on islands.

Yes that would work, and you can land the same turn.