With GS I have seen plenty of AI air and naval units and they use them. Sure, the AI doesn't use any units unless they have a plan, so the AI won't do pot shots just because it's possible (like most players do), but they (now) use them. Then how much air and naval units an AI have depends on what they like, different civ like different things. In my latest game (playing Canada), I was joining in, in a lot of emergencies and in one of the late era ones, America had a whole lot of planes and bombed Korea back to stone age (made it much easier for me to swoop in and take the city (that was the emergency)). In my first GS game, I was attacked by India with a big battleship navy. I didn't had much of a fleet myself (I had never needed one before in Civ 6) but was saved by my neighbour and ally Phoenicia (with the biggest navy I have ever seen in civ 6).