Which greek philosopher explained that water is the essence of all matter and how do we know about his teachings

The answer is Thales of Miletus --- an early Greek physicist.

ARISTOTLE:
"Of the first philosophers, then, most thought the principles which were of the nature of matter were the only principles of all things. That of which all things that are consist, the first from which they come to be, the last into which they are resolved (the substance remaining, but changing in its modifications), this they say is the element and this the principle of things, and therefore they think nothing is either generated or destroyed, since this sort of entity is always conserved, as we say Socrates neither comes to be absolutely when he comes to be beautiful or musical, nor ceases to be when loses these characteristics, because the substratum, Socrates himself remains. just so they say nothing else comes to be or ceases to be; for there must be some entity --- either one or more than one --- from which all other things come to be, it being conserved.

"Yet they do not all agree as to the number and the nature of these principles. THALES, the founder of this type of philosophy, says the principle is WATER (for which reason he declared that the earth rests on water), getting the notion perhaps from seeing that the nutriment of all things is moist, and that heat itself is generated from the moist and kept alive by it (and that from which they come to be is a principle of all things). He got his notion from this fact, and from the fact that the seeds of all things have a moist nature, and that water is the origin of the nature of moist things. [Metaphysics ; BK I, Ch. 3. 2nd and 3rd paragraphs.]

Themistocles was an Athenian general/strategus who didn't write much, but was famous for his defence of Athens in the 1st Persian war of about 490 B.C. Solon was a leading Athenian politician, in the "golden age", after the 2nd Persian war (480 - 479 B.C.) and the Athenians (rather than Alexandrians) had the records of his legislation and speeches, since he was one of the most famous Athenian "archons". Homer long predates Plato and hence wrote nothing about Plato in the Iliad (circa 600 B.C.). Plato was in his 20-es when Socrates was ordered to drink Hemlock in 399 B.C., so Plato comes a long time after Homer.

Herodotus was an early Greek historian who may have written about Thales. And, as above quoted, Aristotle certainly wrote about Thales in his Metaphysics (primary philosophy) treatise (circa 330 B.C.)