S. R. Faruqi:

This verse has been retained in SSA [shiʿr-e shor-angez] only to make the form of the ghazal; otherwise, the verse is commonplace. With 'fountain' there was no need to say 'of water'. In the line there are many words of padding [bhartī]. Here koʾī has been used as one long syllable; in Mir's time this was not improper. The wordplay of chashm and chashmah is obvious. And then, from this perspective the verse is not devoid of interest, that in it the beloved's weeping has been mentioned.

It's also interesting that although the word 'fountain' in itself conveys the idea of water ( chashmah can mean a small river, a fountain, or a stream), the word pānī is usually used with it. Thus Iqbal too, in a peerless verse, has not been able to avoid this. A famous verse of ḳhiẓr-e rāh is:

aur vuh pānī ke chashme par muqām-e kāravāñ
ahl-e īmāñ jis t̤araḥ jinnat meñ gird-e salsabīl

[and those haltings of the caravan at fountains of water
the way in Paradise the people of faith surround Salsabil]

For the way Nasir Kazmi has used the image of the weeping of the fountain, it would be hard to find a peer:

andherī shām ke parde meñ chhup kar
kise rotī hai chashme kī ravānī

[having hidden in the pardah of the dark night
for whom does the flowingness of the fountain weep?]

FWP:

SETS
MOTIFS == EYES
NAMES
TERMS == PADDING

I don't see why the weeping has to be the beloved's. In fact that possibility never occurred to me until I read SRF's commentary. Why can't the surging fountain memorialize some lover's weeping, as would be much more in the traditional ghazal vein? So I asked SRF, and he replied (Jan 2018):

'Arguably, it could be any eye, any lover's eye. It cannot be his own eye, because speaks of kisū chashm . But any eye, or a lover's eye, is quite insipid. Stating the obvious, in an obvious way. So it should preferably be the beloved's eye: maybe she was really sad at the lover's going away, or somebody’s going away.'

Note for meter fans: Here's an example where koʾī has to be scanned as one long syllable. This is rare in later poets, but Mir does it freely.