As screenshot #1 shows, when you're NOT considered in water, edges of the blocks show. However, going down 0.001 block (which makes the game consider I'm now underwater) makes these edges disappear, as you can see in screenshot #2.

So, of course, building under water can be a real pain sometimes (especially for blocks whose hitbox is different from what we see, like for kelp : unless being right above and looking 90 degrees down, it's hard to know where to aim when not seeing the hitbox).

Note : I'm not sure how I could be the first to talk about such an easy-to-spot bug, but I couldn't find anything about that, so...