S. R. Faruqi:

āp meñ = inside oneself

It's an entirely new image, and a good example of illusion-creating imagination. A whirlpool constantly keeps rotating, thus he's assumed that it's sacrificing itself by going around itself. But why has the whirlpool become a lover of itself? Perhaps because the whirlpool has seen within itself that ocean of beauty, of which it is a single part, displaying its glory/appearance.

Maulana-e Rum, in the 'Masnavi', has recorded [in Persian] a story that once Hazrat Bayazid was going on the Haj, when on the road he met an elder. The elder [buzurg] said to him, 'You are going to circumambulate the Ka'bah? Allah one time called the Ka'bah his house. Myself (that is, the human heart and existence) he has seventy times called his slave. Circumambulate me!' In a translation by Qazi Sajjad Husain:

'Although the Ka'bah is the house of His worship,
My existence too is a house of His mysteries.
Since he made that house, I have not gone into it.
And in this house, no one has entered except that Life.
Open your eyes well, look at me,
So that you will see Allah's light in human form.
The Friend one time called the Ka'bah "my house."
Seventy times he has said to me "Oh my slave!"'

Thus the elder ordered Hazrat Bayazid,

'He said, "Circumambulate me seven times,
And consider that better than the circumambulation of the Haj".'

It's probable that the fundamental theme, which can be found in various different forms among the Sufis, Mir would have taken from Maulana-e Rum himself.

But the image of the ocean and the whirlpool is his own, and it's that image that's the basis of Mir's individual glory. Then, there's also the fact that Mir's insha'iyah, interrogative, self-addressed kind of style has raised this verse to the level of revelation [kashf].

FWP:

SETS == MUSHAIRAH
MOTIFS
NAMES
TERMS

The whole somewhat confusing first line might become, in modern prose phrasing and order: kyā [ kisī ne ] us bahr-e ḥusn ko apne āp meñ dekhā hai ? Only at the very end of the second line do we discover what the subject is: the whirlpool. Thus we have the makings of a 'mushairah verse', with its punch-word withheld till the last possible moment, then suddenly shedding light on the whole verse.

And speaking of moments, SRF hasn't mentioned what I think is the best part of the verse: the excellent wordplay of laḥz̤ah laḥz̤ah , which means literally 'moment'-- but also 'look, glance'; its close relative laḥz̤ means something like 'a sidelong glance' (see the definitions above). So the whirlpool has 'seen' inside itself the (divine?) beloved's absolute beauty at every moment-- or, equally, at every look/glance. And perhaps the beauty that it sees includes the 'sidelong glances' of the beloved herself? (Perhaps they even become, or appear, sidelong because of the whirlpool's constant whirling?) And can an 'ocean' of beauty even fit inside a 'whirlpool'? (No wonder the glances can only be sidelong.)

Note for meter fans: In ke taʾīñ (an archaic form of ko ) we have to scan taʾīñ as a single long syllable; this is a permissible variant scansion.