Okay, if I understand correctly (which I'm not sure I do), you should use Data Driven Pages to cycle through your lakes (it will make one map for each lake). Set up your symbology (once) in the map. Then, export all your maps. If that's not at all what you want, please elaborate. You can't set different definition queries within the same feature class at the same time. You script seems to read through each feature changing the definition query on the layer each time. Essentially, you'll be left with with the last definition query, if it works at all.
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