Why does cooling water exit above waterline?

[MarkI]

MY AC/heater uses sea water for cooling/heating transfer. The sea water enters a through hull, goes through a pump, heat exchanger, and then all the way aft to an above the waterline exit (through hull). There are threee such exits with a lot of hose in a new boat, there has to be some reason.

So when the heater is on (winter, or AC in summer), there is a constant splashing, that makes much more outside noise than the generator. It makes me shy to use heating when at a marina, or rafted up, or even perhaps at anchor (which I have not done yet).

Why don't they just exit the water out a nearby through hull that is there anyway to for instance source water for the head? It would be quieter, AND use less plumbing (this is a new boat design, 2019).

Is there a reason not to reroute the water exit to a nearby through hull (that is spaced away from inlet thru hull)? I realize it would be harder to confirm the pump is working, so I would use a Y valve to allow routing to original exit.

I am worried there is some problem with a slat water LOOP, as I have found no other such loop. Engine coolant goes through an air separation via its mixing with exhaust, same for generator, and head uses go through either black water tank, or sink drain.

Would I create some sort of corrosion path if I have a salt water loop? The pumps appear to be centrifugal, and would not separate the water path.