There are definitely lakes in East Fishkill but I wouldn't suspect you could go waterskiing in any of them. They're pretty small; for example, Red Wing Park has a lake which is available for swimming, but it's not big and I think waterskiing would be verboten.
If you're going to go waterskiing anywhere around here, I think my first suggestion would be the Hudson River. It doesn't run through East Fishkill so you'd have to haul over to the river, but it'd probably be more worth your while than any of these small lakes which probably don't allow it. I also know that people can do it there because I've witnessed it, so that's probably the best option. It's not a lake, but it's my best suggestion.
