What is the source of the word "Shamayim"?
Rashi: 'Sa Mayim' (it carries water), or 'Sham Mayim' (there is water there), or 'Eish u'Mayim' (which Hashem mixed together to form the Heaven). 1
The Ramban explains that Rashi follows his earlier explanation (See Pasuk 6), in which case the Shamayim here is the same Shamayim as that mentioned in Pasuk 1. The Ramban himself takes on that the Shamayim mentioned there is the Heaven above the 'Chariot' - about which the Torah offers no details (due to its spiritual nature - just as it does not describe the angels). The Shamayim here on the other hand, refers to the sky (the celestial sphere with which we are acquainted). From now on, the first Shamayim is referred to as 'Shmei Shamayim'. The Rashbam too, makes the same point.