I went and read the complaint letters from the WI (and MN) environmental groups, and the complaints broke down into two areas:
1. The diversion violates the interstate compact, which says that significant diversions are supposed to have some public or residential benefit. Foxconn's doesn't. It's not inherently problematic for the environment, but it sets a precedent they don't like.
2. Foxconn might pollute Lake Michigan.
That second concern is of course valid, but has nothing to do with the amount of water Foxconn consumes. I found one complaint that claimed consumptive use of water (that is, unlike NIPSCO, water that is diverted from the lake and not fed immediately back) could cause long-term problems because Michigan only replenishes 1% of its water annually. But Foxconn's draw is something like 0.0000002%.
If the lake was ringed with Foxconns from Gary to Mackinaw City, I'd see the problem. And: totally legit to be vigilant about precedent! But I don't see the intrinsic practical impact from Foxconn today (in terms of water usage).
Worth noting: Foxconn came back and cut their water demand in half with a recycling plant. Not that I want to sound like I'm defending Foxconn, because I think this is a really bad deal for Wisconsin.