Re: What are these stern holes for?
How could he find out for sure ????
I understand your irritation with our guesses, but they are at least somewhat educated guesses. The left one looks like a 3/4" livewell pump thru-hull, and since it appears to be plastic, probably it is part of the pump itself and not installed with a sea ****. (If so, I think this is a Very Bad Thing. Livewell pump tiny screws strip or housings crack and fail more often than drain hoses, so I believe livewell pumps below the waterline should always be backed up by a sea ****.)
This is just an assumption and was just meant as a suggestion as to what the OP should be looking for.
Tracing the hoses is often not feasible. That's why I suggested draining his livewells and watching what happens. I should have pointed out that there may be other drains he has not considered, such as the anchor well drain, any drains under the cockpit, any built-in cooler or fish box drains, etc.
As to how he could find out for sure? Here's what I'd do, while the boat was on a trailer:
1. As Don says, try to visually track the hoses attached to each drain. If that doesn't work....
2. Dump some water into anything with a drain. Livewell, anchor locker, fish locker, cooler, under the console, battery box, wherever. Note where it comes out. If that doesn't work...
3. Get a friend. Position friend inside boat. Go outside the boat to the transom holes. Apply mouth to transom holes and make trumpet noises. Ask friend where he hears sound coming from.
This was an excellent question by the OP. Every boat owner should be aware of and fully understand every hole in his boat below the waterline. You are correct that he should find out for himself, and not rely on internet guesses.