all of the existing holes are EASY to figure out.... MUCH easier than installing an a/c system... you have drains for sinks, bilge pumps, maybe anchor locker drains, etc.... they are generally near the thing they are draining.
If the boat has not already had an a/c system you WILL need to drill for the discharge... do NOT try to T intxampleo an existing discharge unless you like the idea of your boat sinking.... Example: you T into the galley sink drain, you get an obstruction in the thru hull and the sink begins overflowing.
It gets it's own thru-hull