Our sink is exactly like the one that Commander Pete has posted pictures of.

We normally do not use the sink when just daysailing or staying overnight at the dock. But when we are cruising and living on the hook it gets it use. Brushing teeth, washing hands, making coffee, water for cooking, rinsing food. We don't wash dishes in there as it is too small, I carry dishpans and do those in the cockpit, using dawn dish detergent, and we toss the "brown" water overboard.
If you need to drill a "new" hole for the draining of the sink, I would put an actual thru-hull on for safety. That way you can close it when not at the boat, sailing, or just because you are not using the sink. I do have a picture of the thru-hull we installed for an intake on the head in the forward v-berth in the gallery.
I thought I had posted pictures in the gallery last year, but that was of the conversion we did installing a head and making the old water tank into a holding tank. I will take a picture of our "new" flexible holding tank and will do any measurements that I can get for you.

Have a great weekend!