Currently, the hotkey can be used to trigger flight regardless of the environment. To me, this feels odd, as it allows you to lean forward and enter flight whilst, sat in a pool of water, or stood in a confined space with no room to physically move. To counteract this behaviour, an option could be added to require the player to be in the air (so out of liquid, and either jumping or falling). This solution isn't perfect, and I'd be happy to discuss other ways to combat this "issue".

Current version: 1.1.2 for Minecraft 1.12.2