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Soon one of our Venture 23 owners will probably answer your question. That is usually a bilge access. I have never seen a clear one, but that is fine--you can check for water or access with a pump. I am not sure that is the lowest point in your bilge when boat at rest or on a plane. These are the two places that bilge pumps should be placed. There may be an area with a depression as a "bilge pump sump" This area may only have 1/2" core or it may have no core.