I think Winni probably has every size of rock known to man. From fine sand up to what might be house size. Hard to tell on sonar if those biggies are freestanding or outcrops of bedrock. Probably a mix. They are mostly better worn & rounded vs the Minnesota sharp edged lure eaters of my earlier days. I've lost more jigs to rocks in a day of fishing there than I have in 3 seasons on Winni. Same goes for anchoring. Seems to be more sand and gravel filling in the gaps here which helps prevent anchor eaters though I am sure there is a wealth of lost ones in Winni. So, maybe you can leave the anchors at home and use snorkel gear to "shop" for one here?
Oh, the only place I've had good luck with navy anchors was in seaweed beds or mud flats.