Your sinks drain into the South Bay...
Don't know what year your boat is or if all Catalina 30's are the same.
Here is a picture of my engine bay. This is directly below the sink cabinet. The sink through hull is right next to the enigne's muffler and is accessable via the hatch across the ladder.
Enclosed are some pictures. They are pictures of my boat when I bought it. The gate valve is gone and the other 2 pics show the lower hatch I am taking about that gives you access to the through hull.
But butchrobbins is right, you've never looked around inside your boat?