As the Zohar states in Bereshit A, 7:
“After the point and the chamber were established as one, Beresheet - WHICH IS ARICH ANPIN - includes a lofty beginning to THE LIGHT OF Chochmah. Later, the appearance of the chamber changed and it was called a house (Heb. bayit) and the supernal point was called the head (Heb. rosh). They were included in each other through the secret of the Beginning, BECAUSE COMBINING THE WORDS BAYIT AND ROSH FORMS THE TERM BERESHEET.”
I add the following:
בית = Bayit = House;
ראש = Rosh = Head;
בראשית = In the beginning
And there is fire in ‘Bereshit’ for ‘esh’ is in the the center.
So we see how the Head is within the House of Creation and if we look within the Head there is Chochmah which is represented by ‘esh’ fire within Rosh. And we know that the first of sayings, ‘Bereshit’ is Chochmah.
ראש = Head; Beginning
אש = Fire; Spark of Chochmah
So in the Beginning when G-d created HaShamayim they were one as Shamayim.
ש = shortened form of ‘esh’ Fire.
מים = Waters
And why is the ‘esh’ in Shamayim missing the Aleph? It could be grammatical, but I think because the Spirit of Elohim (Represented by the Aleph) brooded over the waters.