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...
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I also noticed that in this snapshot
Thank you for your report!
However, this issue is a Duplicate of MC-9553.
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@Bemoty
No, this is not a duplicate of the issue you linked. The issue I'm talking about is that you can see the block's hitbox, no matter if they are out are in water, as long as YOU are out of water. And once you are underwater, you can't see anymore ANY block hitbox, including from blocks that are out of water.
Also, as I already said in my report : " 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...". But I was saying that because this is a new bug since 1.13 snapshots. There wasn't that issue before. And, once again, it is not a duplicate from the issue you linked. I'm not talking here about a barely visible hitbox, I'm talking about the fact that they do not show at all.
My other bug report, MC-128066 , that I made separately for a reason, looks much more like yours. This time it could be the same bug, though in your bug report there seem to be also other things completely different from that (but it does look a lot like what shows the screenshot with a barrier block. Also, once again, I did try to check if there was already a bug report. Not my fault if I can't always find them. Keep in mind you never saw the bug reports I never made since I found people already talking about them). You can then say my other bug report is a duplicate if you want, but this one isn't. So please, open it again. Thanks.

@Bemoty
Except that here, the hitbox does not render at all. And please, read my whole message this time, because I have really good reasons to think you didn't the two previous times (or that you did by not paying attention to what I was saying). Thanks in advance.
Similar consequences don't necessarily have the same causes. That's as true for bugs than it is for diseases, psychological problems, or computer issues. And here that's not even a similar consequence. This is not the fact that there's water between us and what we look that causes the problem, it's the fact that we ARE in water, looking in water while not being in water isn't a problem.
Also, the fact that other reports have been considered as duplicate doesn't mean the people who did that were right. The only thing I've learned with those reports is that the bug is actually older than what I though. And that's because 1.13 will make building in water much more popular, so this bug needs to be fixed for 1.13. So it would be a much better idea to give it some light. And that's not even a builder saying that, just someone that thinks about others (all I personally did in water were tests and I'll probably never do anything else than that).