and have experienced similar. A few years back, I was running somewhere at Silver Lake, cruising along. Marsh was way up ahead, I was just running on leaves, then WOOSH waist deep, very brisk water. Take about a shock! Completely indiscernible until I sank. I wish my memory was better to know if it was in the same spot or not.
It was the extreme eastern tip of the marsh that is just west of C15 on the red course (75?).