It is my belief (based on testing on version 1.21.1) that higher levels of Aqua Affinity results in higher mining speeds underwater, potentially leading to underwater mining being more efficient than normal mining, if higher levels of aqua affinity are available.
I tested with netherite pickaxe with eff 8 (because this matches the one I'm using on the server I play on) with different levels of aqua affinity, mining obsidian underwater. In the same timespan, I gained increasingly more obsidian with higher levels of aqua affinity.
Some more rigourous testing is required to map out the details of the benefit. Feel free to reach out to me: Minoxsta, on Discord, if you would like my help to perform some tests or something.
