Now I have to find it too?
Ok. Buy a $19 smart phone endoscope camera from Amazon. If you don't have one, get a high candle power flashlight. Take boat to the dock. Don't use the hose connection if you have one, because it might bypass things like your Y pipe, if you have one. Put the boat in the water. Warm it all the way up so that the thermostats open. Stick flashlight and camera in the bilge, tracing the intake hose from your transom assembly to the water pump and block, up to the manifolds, and out to the transom assembly. One side will be wetter than the other at first. Pay close attention to the exhaust risers. Look for light colored calcification stains.
Confirm it is not coming in at transom. Super double check.
Because you only leak on plane with engine running: Cooling system; always suspect exhaust riser gaskets and hoses.
Because you can't see it easily: Suspect something on bottom, between some things, or aft.
If you only leaked at idle/when moored: Drive bellows, shift bellows, drain plug, hole in boat, other fitting below waterline, bravo block-off plate if through hull exhaust, transom assembly bracket, etc.
When you can tell what is leaking, take a pic with your phone and post it.
