I am not an expert in Arabic, but I have discussed this question with a scholar and according to him, the answer is following.
The confusion you raised is created due to reading only the translation and not the original Arabic text. Although a translation of any text helps to understand the general meaning, but can never exchange the true meaning and tone of the original content of any language. The same went here in this case.
For the verse 21:30, that is
And that We have made every living thing out of water
used Arabic word جعلنا which has root word as جعل. Its meanings also include ‘to cause’, ‘to bring about’. The whole verse means that Allah has caused every living thing to depend on water. In other words, He has made continuity of every life hooked with some form of water.
In the other verse, that is for 15:27 the translation reads as
...and the jinn We had created before from flaming fire.
Here Quran used word خَلَقۡنٰهُ for which the root word is خلق. This means “to create” or “to make”. Here, Allah is talking about the material of creation. Similarly, just one verse before it, i.e., at 15:26, Allah discussed the material of the creation of man, which is clay.
We created man out of dry clay, from molded mud
So, reading all these verses with their correct meanings known does not create any confusion. Collectively reading all these verses enables us to conclude that Allah created Jinns from the fire, human from clay, and made all His living creations dependent on water. Now, as humans, we know that our life is dependent on water. Since we have never observed or perceive through our senses the living style of Jinns, therefore we do not understand how their life is dependent on water. But since the Holy Quran said this, we believe that what is said in it is like it is said.
MaShaAllah! Very good question. This shows that you are reading the Holy Quran with thinking. May Allah makes this a source of guidance for you, me, and everyone else who is reading it