I didn't even realize this wasn't available, and now I'm worried when doing more geographic aspects of the world without these categories.
There are so many different names for geographical phenomena that I feel it should almost be a write-in, with an (Underwater) or (Flying) or (Space) tag. All the way from planets to islands to continents, there's too many to put in a drop down. I'm not sure if this is a feature already but you should probably be able to write in [Atoll] and give it (Underwater) tag, as opposed to giving every type three possible subtypes which is what this seems to be suggesting. My suggestion to this would be to get rid of the current dropdown and have one for tags instead. For example: I make the Continent of Not-Australia. I would write "Continent" for its type and dropdown would have several options for tags: (Underwater), (Flying), (Space), (Extraplanar) or leave it blank for standard type.
also building on Scalenex's suggestion, maybe a checkbox for (floating) for things like floating mountains or flying cities
I agree with Scalenex, but otherwise I 100% support this decision!
rather than have a single underwater category, I would prefer a check box that adds (underwater) to any existing descriptor. That way you could put a forest or a city or a mountain or anything else underwater with a click of a mouse.
being able to specify places being underwater would definitely help