An alternate suggestion: If you're willing to give up the overboard discharge from the waste tank (but keep deck pump out), you could use that seacock as your dedicated cooling water and only open the seacock when you run the AC.
The factory option usually ran a hose all the back to the bilge, either to a dedicated seacock, or tied in to an existing seacock. But that is a long run and I think loses a fair bit of efficiency of cooling water.
You would have no need for the pump that sits on top of the waste tank.
For the discharge of the cooling water, it is much easier, and a less scarier, to cut a new overboard discharge hole in the side of the boat above the waterline. I'm not so confident that the sink drain could handle the cooling water discharge volume. It would depend on what size cooling water pump you go with. And I'll suggest going with the biggest pump the manufacturer recommends for the most efficient cooling power of the AC.
The drip pan of the AC is a slow trickle and that can be routed to the shower sump pump, especially if it's all a downhill run. If you use your shower, now may be a good time to make sure the sump pump is clear of debris and hair. You should be able to manually test the float switch, too.
And as I learned from this forum during my install, obviously double clamp all hose connections. And when you do, make a point to face the clamps in opposite directions.
Also, the cooling water pump is likely to pick up some air from beneath the seacock. Especially mine, which is rack stored, so a pocket of air is trapped beneath the seacock every time the boat is put in the water. It tends to cause an air lock in the cooling water pump.
My solution was to place a 'Y' valve after the cooling water pump with one short hose leading to the shower sump. Now, before I start the AC, I open the cooling water seacock with the 'Y' valve completely closed. Then I briefly open the one hose path leading to the shower sump. That seems to clear the air from the pump. Then I can close the sump pump side of the 'Y' valve and open the AC side of the 'Y' valve and I'm ready to crank on the AC.
Obviously I always check to make sure there is good overboard cooling water flow coming out of the side of the boat.
Last thought: A 4200 BTU unit might be a little light if you boat in some really hot climates.
I have a 6500 BTU unit on my Sailfish and it does an okay job here in South Florida. I also carry a simple, fold-up canopy that I can hang over portions of the cabin in direct sunlight and that helps.
In my year (2007) Sailfish, the factory option was for a 7,000 BTU unit. You may want to check the Grady catalog for your year 265 to see what the factory was using on your boat.
Good luck with your install. And post some pics along the way!
Rob