I think Cedar Fair still operates Dorney Park & Wildwater Kingdom under a one admission policy, that is one ticket gets you into both rather than having the water park be a seperate gated facility.
Also, given the layout of MIchigan's Adventure, it's hard to see how the water park could be made a seperate gate. Not only would it cut off access around the small lake the park surrounds, but a seperate gate for the water park would involve demolishing and moving the corporate hq, hr, receiving, employee parking and other 'behind the scenes' things.
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Time flies like an arrow. Fruit flies like a banana.
--Groucho Marx