Innerkip is well worth a visit if you live within an hour or two drive.
But the time to go is June. In May, the viz is OK, and the suckers are spawning, which is kind of interesting. It clears up in late May to early June, to the point where, after a few days of good weather, the viz is 15 feet--sometimes even 25 feet.
There are lots of fish: rock bass, smallmouth bass, suckers, pike. There are also several old boats, buses, and cars, and there is a map around with compass bearings and fin kicks that you can use to navigate the bottom.
Then after about the first week of July, the viz starts to drop, and by September, it can be really bad, e.g., 2 feet. It clears up in November, but not as well as the spring. I have never tried diving there later than that.