Mining downwards while underwater shows animation but does not break block

While standing on a block and mining an adjacent block, on air, while standing, you can reach 3 blocks down. While crouching, it's 4 blocks. However, if you go underwater and go into swim mode, you can target the 5th block, and it'll show the breaking animation, regardless of mining speed, however the block will break and be instantly replaced.

This doesn't give the block, or seems to consume durability on your tools, but it does look funny, especially if you can insta-mine the block