big ol' catfish or lake sturgeon in Big Cedar?

[smoothfin]

Had a somewhat murky dive at Big Cedar Lake, viz was a cloudy 12', and I had temperature in the upper 40s,  parked in the Linden restaurant lot (which may be out of business, according to a disgruntled coupon-holder), and entered by their piers.  On the surface swim out to deeper water I saw the biggest fish I've ever seen in freshwater, looked to be all of 5' and maybe a bit more.  I think we were in about 6- 8' of water at the time and it was swimming under me.  My dive buddy also saw it, and he can confirm that I hadn't been into the mushrooms yet.  Our current opinion is that it was a catfish.  we quickly ruled out musky and northern and carp.  My view was from the top down. The color looked to be grayish blue, smooth skin, not a terribly wide head, and it did have some whiskers.  Kind of fits a description of a channel cat, but I don't know if they get that large or not.
Here's a dnr pdf on catfish that describes the channel cat vs flathead, and it mentions that channel cats don't get as large as flatheads:
The other possibility is that it was a lake sturgeon, which might make more sense given the size, and the coloration still seems to fit (), and the head of the fish seemed to also fit the description, except the body looked more smooth than what the sturgeon seems to have.
Anyone have any personal experience with really big catfish or lake sturgeon in Wisconsin waters?