... Notice that he drank an excssive amount of water, which is a symptom/
Is this in addition to the 'heavy smoking'???
I haven't viewed GFII in a while, will have to check out the next time for how many times we see Michael Corleone with a glass of water in front of him. Which of course, would indicate 'excessive' drinking of same.
The diabetes in GFIII was most likely written in as another way of having the older Michael come off as more weak and vulnerable, nearer the end of his effective reign as 'Don'.
And I agree that it was no more than aspirin he was downing on that train in GFII.