IF your hunting the rubble wall you'll find your perch and lots of carp. If you move to the north and hit the solid wall, by the south gap, you'll start running into walleye's and such.
Water temps will climb in august and the winds will make a huge difference in clarity and temps. You'll also find a difference in temperature between the inside of the wall and the outside of the wall. It's not uncommon to get water as warm as 75 degrees on the inside of the wall due to the shallow nature of the area and the restriction of current to move the warm water out.
When you get a strong east wind for a couple of days it will get pretty muddy at first, which is also the best time to surf the area- with a long-board and everything.
After about a day to lay down you'll notice the water is much warmer, all the way down past 100' many times, and a bit murkier. The murky water I'm talking about is nothing like it was 20 years ago- the zebra mussels took care of that. It will still be a good 20-30', but it's not going to be the super clear stuff we get when the cool water blow in- over 100' of vis many times. Expect temps to hover pretty close to 40F when the really clear water comes in.
I'm not sure on the legality of your spearing perch in Lake Michigan, but be aware that the DNR, and the university, net perch very close to the area that your diving in. They do this for sampling purposes as the thought the population was dying out a couple of years ago and have since been trying to re-stock. If you get out close to the green bell buoy you may see where they have them set. If they don't have them set I would work the area of that green buoy on in to the southern tip of the rubble wall- there's a rock reef that runs that whole area. The bottom on the south side of it bottoms out at 40' and the top of the wall will hit less than 10'. The North side of the reef will vary between 20' and 30'. I always see lots of fish in that area- as well as the DNR nets.
A 5mm suit will should be more than enough for all your diving around here- even under the ice. If you want to try and dive Lake Michigan without a suit you can in about another month.
Jon