Nothing does. That entire system of the transom mount exhaust housing and the exhaust Y pipe are full of water up to a level about even with the bottom edge of the exhaust manifolds when the boat is in the water. This is called static water line. Also why some engines suffer water ingestion and need taller risers to get the exhaust elbows high enough to prevent that.
When you pull the boat out most of that water drains via the holes in the exhaust bellows but not all of it (bottom of the exhaust housing is lower than the exhaust bellows). The exhaust manifolds remain full of water which is why they must be drained during winterization.
