What do you call the part of a lake that is circled in this picture? I keep thinking "lagoon" but lagoons are salt water and coastal......
This was the first word that jumped into my head: inlet. Turns out that's correct—although there are probably other names as well.
I think inlet would work, although in my mind they're more linear, less rounded... I think I'd call this a bay. And, yes, lakes can have bays. Some lakes have lots of bays and are named after them... https://www.lakeofbays.on.ca/en/index.aspx
I would say it's a bay, though the mouth is almost too narrow. Though sometimes a sandbar can form across part of the mouth of the bay. It looks like it could also be described as a harbor, either natural or artificially-dredged. How important is the technical title anyway? Can't you just call it something like "a narrow hook of water opening into a wider pool" or something like that?
I've seen it described as a "barmouth bay." But I doubt the casual reader would recognize (or care about) those labels.