I'd think most algae, bacteria, aquatic insects, etc. would not survive the lye, but they could still produce impurities in your soap. I would be equally concerned about the mineral content and what that might do. So either way, filtering is a must for reducing chance of DOS -- plus you'll be glad you can look customers in the eye when they ask if you filtered it or cringe at the thought of bathing in lake water. I'm the kind of gal that would be interested in a soap made with the lake water, though I'd guess I'm in the minority. But I would definitely avoid any green coloration or fleck-producing additives to discourage people from thinking about algae or other organisms in there (since most people probably would be hesitant about this). Blue would be good though, or even natural color or white. Another idea would be to make a fisherman's soap -- people might really like the idea of it bringing good luck for a bite! Even if it's just for a novelty.
