I looked at the Hiragana on Pixiv and it's actually "ama ama" あまあまの水さん, ama-ama no Mizu-San – I looked up "ama ama" on Jisho and it gave me this:
甘々(あまあま ama ama) – Adjective, sweet;sugary
水さん is "Mizu-San," so final translation would be "Mr. Sweet-Sweet-Water" or "Mr. Sugar-Water."
I read all the (machine-translated) comments (on the Pixiv page), and I think this is what everybody was trying to explain:
If the KoMarisa had not been contaminated by the thought that the fourth drop of water would be sweet, there wouldn't have been any problem with drinking the water as usual, but because somebody (presumably a human) told her that the fourth drop of water would be sweet, this created a doomed scenario for two separate, yet interrelated reasons (it's kinda confusing but I'll try my best to explain it clearly):
1. Because Yukkuri / KoYukkuri are generally unable to count to the number 4 2. (and I have to add this second reason, because it would explain why the KoMarisa died just from drinking ordinary tap water, because reason number 1 does not fully explain why the KoMarisa spontaneously died) Because it gave her the impression that the three other drops of water would for some reason, through the KoMarisa's own logic and reasoning, actually be poisonous, and not just normal ordinary tap water
And to end reason number 2 and connect it back to reason number 1, because a KoYukkuri generally can't count to the number 4, this created some sort of malfunction / infinite-loop in the KoMarisa's mind where every single drop would have become poisonous since her mind is unable to reach the number 4 – thus, each drop of water became poisonous (again, this doesn't quite fully explain why she even thought the other drops would suddenly become poisonous / why she wouldn't just drink the other 3 drops of water as just normal water instead of dying spontaneously, but this is the only thing I could come up with that makes sense).
So I believe Yundoru-San meant to portray the KoMarisa as having been fooled by an external thought that was implanted into her mind (every fourth drop of water is Mister Sugarwater), which the KoMarisa subsequently, somehow, in some strange mysterious way, reinterpreted using her own KoYukkuri line of reasoning to mean that all the other drops would be poisonous.
tl;dr KoMarisa kills itself through its own belief (although the way in which it was portrayed was, if I may say so myself, a bit rather convoluted and hard to follow)