Yes, you can. There's docks for boat lunch that you can walk down to. Not so good for swimming however. But if you go on the other side of the dam there's a lil turn out and you can walk down to the water from there. Its a short distance.
Not at the marina, but if you go around the lake to dark day day use area you can access the water without a boat. On a side note the marina does have boat rentals.
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