We've only camped there overnight twice and didn't swim as the two stays were in October and the water was too cold. I do remember a small beach like area that looked swimable near the very southern entrance end of the CG area but not at the campsites as it's vegetation to the lake's edge. I don't remember clearly what the bottom was right at the campsites though. I do seem to remember some stony and sand bottom at the little beach but there could be rocks on the bottom in the deeper water,just guessing.
Lake Gogebic runs about north and south, is quite large but not very deep by Michigan inland lake standards. About 13,400 acres in size, 2-1/2 miles wide and 14 miles long but only 38 feet deep at max. A very pretty area. Good fishing in the lake with walleyes and northern pike being the largest fish and smallmouth bass being the next largest. Remember that northern pike have "Y" shaped bones that can get caught in your throat and cause you to choke so ask locally how the clean well and remove the Y bones. Walleye's are an excellent eating very mild white meat fish.
Have fun!
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A like new '07 Sunline Solaris 26' TT