Not sure what you're talking about...?? I have the "Horizon" fresh water cooled engine...it has an air pump to open valves to drian the raw water side. My older engine had the blue plugs you had to manually unscrew to drain the block. Can you post a photo.....or you can go to
www.go-fast.com with you engine serial number and do a serach for parts with pictures and get the cooling system diagram. Definitely get a merc factory service manual......