I do think it would be fine to "T" off the engine raw-water intake to feed the A/C, especially if you'll only be using the A/C on shore power.
If, though, you have a generator to power the A/C and plan to run it and the main engine together I suppose there could be insufficient water flow for both and you'd want a separate thru-hull for the A/C.
In my case a prior owner had already installed another thru-hull for a water maker and I just ran a "T" off that for the A/C cooling water.
He also installed a 6kw generator (with dedicated thru-hull) and we have on very rare occasions run the generator for A/C while we were motoring up the ICW in oppressive summer heat (and actually ran it all night at anchor - yikes, we're a trawler!)
So I've got 8 thru-hulls for: 1. Main engine, 2. Generator, 3. A/C & Water maker combined and 4. Refrigeration. Plus, of course, 5 & 6. Toilet intakes and 7 & 8. Toilet discharges.
Good luck with it,