In a 23 inboard it makes perfect sense. They are the engine intakes. That is how the engine breathes. I would also bet at least one of those vents, probably one of the stern vents has a hose going down into the bilge below the engine oil pan and it's attached to a blower. I am reasonably certain that is why they are on the 20 and 23 IO's. Just adding the vents isn't anywhere near adequate to vent a hull from fuel vapor with out a blower. Especially when it matters. AT START UP after a boat has been sitting and the vapors have had time to accumlate. The blowers are turned on for a few minutes before start up and turned off after the engines fire. Once the boat is underway all the airflow necessary to vent the hull is created through the engine intakes.
My guess is they are there because of the inboard models and they just kept the line looking the same across the board.
If you have an inboard boat move the vents to the Center Console or into your liner. It will accomplich the same task and your hull will look cleaner. I/O guys move them to the engine cover or the liner.
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