My family and I were on Sanibel 10/7 through 10/11, staying at Casa Ybel. The beach and water conditions were as bad as I have seen in all my beach travels. We saw dead fish every day. Not a lot, but probably 5 -10 dead fish every time we went down to the beach. The smell was not the usual ocean / salt smell I'm used to and love. It was an odor that was not particularly pleasant. LOTS of seaweed and a redish junk which I assumed was also a seaweed. Water color was a cloudy brownish green that was different than any ocean I have ever seen.
The best part of our stay was exploring the Ding and the grounds of Casa Ybel. The grounds are wonderful with palm trees, lawns, etc.
We spent less time at the beach than expected due to the beach and ocean conditions.
One last comment. I cannot imagine how crowded Sanible would be during peak season. The traffic on Periwinkle would be miserable and the locals would "roll their eyes" when asked about peak season traffic.
I have had overnight stays in Destin, Pointe Verda Beach, Cocoa Beach, Miami, the Keys, Naples, Bonita Springs and Sanibel. Of all these visits, Sanibel had the worst ocean and beach conditions. I will have a hard time returning to Sanibel. Charming people and the Ding is wonderful. Unfortunately the beach has issues and I suspect they are not short term issues either.