WATER IS SINK.
Evaporating Water is all about manipulating SINK in a number of ways. You use it once to fill in a gap over a river, and a second time to destroy a skull blocking your way to FLAG. I was fine with the second part, but I needed help for the first, and the reason I needed help was because I hadn't properly learned what SINK really meant.
The Witness is about learning a language; you are told nothing about symbols, and through experiments you deduce how they're used. Sometimes you get puzzles that may contradict your earlier assumptions, and you have to rethink the rules you've learned, and see if they can be modified in a way that's consistent with previous behaviour but also opens up new potential. That was a source of much confusion in the The Witness thread way back; I remember a few people suggesting that the rules changed on the player unexpectedly, but that wasn't the case; the rules hadn't changed, you just needed to refine your understanding.
Critical for this, though, is the fact that those puzzles that throw you a curveball - they are universally simple. Critically: Simple enough that you can very rapidly deduce that what you thought you knew has to be impossible for this puzzle to be solvable. Once you can establish that fact, then you can start rethinking your understanding of the rule in question.
In Baba is You, it's similar, with one key difference: We're not working with abstract symbols, we're working with words, which is a double-edged sword. On the one hand the behaviour of the word is already immediately implied by, well, what the word says - but on the other hand, sometimes the behaviour of that word is a bit more nuanced than the immediate interpretation of the word; sometimes you need to ignore what you *think* the word means and instead experiment to confirm what it *actually* means.
The absolute baseline in-game interpretation of SINK is "When Object A has SINK, if Object B occupies the same space, both Object A and B are destroyed".
I think the problem with Evaporating Water is that it expects you to refine your understanding of SINK in a couple of ways. You need to get it to that simple rule above, but I don't think the game leads you in that direction very well. My problems boiled down to not fully-understanding SINK and that I wasn't expecting SINK to destroy text (I believe this is also the first instance where you have to do that?)
Too often prior to Evaporating Water, SINK is used as just a death state. There's only a couple of instances before then where the problem is building a bridge over it - which means there's not much opportunity to *experiment* with interacting text with it. I can imagine a very simple level that would teach it: Four-block wide river with WATER IS SINK, FLAG on the other side, and a Keke in place with a KEKE IS MOVE; the solution boiling down to "Get Keke to walk into the river, then push KEKE IS MOVE after him". You can easily tweak it a bit to also teach the idea of SINK itself being a trait of the mobile entity, just do a touch more to lock in the notion that SINK destroys both objects.
(Brief aside #1: It slightly bothers me that SINK is a more potent destructor than even DEFEAT. SINK applies to anything; DEFEAT - if I'm interpreting its underlying rule correctly - only applies to things which are YOU)
And yet, as I type all this, I have to concede that everything I'm complaining about is stuff I could have deduced with experimentation. So maybe all I *really* want is new concepts to be introduced in a more sandboxy environment where you can experiment freely with objects? More opportunity to *play* with SINK would have been a help. It's what The Witness does - but then again, there's a corollary to that, which is that The Witness only has a fairly small language of symbols when all is done. Baba is You is clearly going to have many more individual rules to comprehend. On paper it gets away with that because the rules are implied by the text on it, but the text isn't necessarily as specific as the player's understanding of the rule needs to be.
(Brief aside #2, a minor niggle: We have MELT and HOT, obviously related. It's mildly irritating that FLOAT and SINK are not related to each other. Again, I guess that's a factor of Baba is You using words that have specific interpretations that imply things rather than abstract symbols - it does make me wonder if Baba is You may struggle in some other languages)