We live nearby and visit Lovers Key State Park (Ft Myers Beach) and Bunch Beach (Ft Myers) and various beaches on Sanibel. We do beach walks and even after a rare cold front which create a strong onshore wind, we collect some Porcelain Crabs, pistol shrimp and tiny sea stars from sponges washed onto the beach. But when we want to snorkel, we go to the Keys. But you are only here for a long weekend so that's not really possible.
You are right about the water conditions here, it commonly low visibility (not terrible, but not like the Keys) and although there are some patch reefs and man-made reefs, they are in deeper water and well off shore. It's possible that is the winds are calm that snorkeling the grass flats off Bunch Beach could be worth doing. We go there with pool cleaning nets and drag the grass flats for grass shrimp which we take home and freeze as food for the fish in our aquariums. We also catch the dwarf seahorses you mentioned. But it's very VERY unlikely you would see them when snorkeling. They can even be hard to pick out when you are searching through the 'stuff' in the dip net for shrimp. BTW, we have also netted big Blue Crab, file fish, other juvenile fish and I once got a foot long porcupine puffer fish that took me 5 minutes to get out of my net!
The shark migration isn't anything I'd be concerned about. I can't remember the last shark bite here on the SW coast of Florida? Maybe 5 years ago a fisherman pulling in a grouper got his hand bit by a shark that was after the grouper. I don't think he even needed stitches. I've been up close to several sharks while snorkeling in the Keys, and likely a few more that I didn't even see, but I've only seen one up close here on the SW coast and that was a 2' long sharp nose shark up near the beach on Sanibel.
Hope that helps? If you have more questions, fire away.