Ninjaed by slothen...As one
can't extend an (in progress) embark, it's moot what lies beyond the edge, and a matter of interpretation.
And/or conscience, if that's a problem.
Personally I more often work with riverless/lakeless surfaces, and avoid aquifers so I don't have too many reasons to need to 'dump' water (and have never purposefully atom-smashed it, in fact find flood controls a little
too water-smashy when I'm working in a very dry environment) but have probably dumped more into caverns (sometimes dry), letting it flow out the edge if it overflows to there before evaporating (or if into an edge-connected cavern-pool, accepted that the water flows off the edge in that circumstance too) and yet
technically if I were to have adopted an additional embark tile or two over the edge that I have chosen I will surely have discovered that the pool is not capable of taking infinite water (for the sake of argument). In that manner it's similarly exploity.
Spreading it out and letting it evaporate might seem the most ethical... Except that surely humidity (or more permanent sogginess of ground, beneath) should occur if every full 7/7ths tile of water has been spread into anything up to seven 1/7th puddles.
On the surface, maybe the evaporation occures without creating too much condensation in a cooler part of the underground fort.
I suppose the best non-exploity thing to do is to pump it into the river or stream flowing through your site (if you have one), and argue that the extra volume
will fit between the banks without creating a marshy bit of additional flood-plane.