The best fried grouper sandwich on the island is at The Sanibel Grill (connected to the Timbers). DH and I aren't big seafood eaters so I can't speak for other places for seafood in particular.
For restaurants in general, Cheeburger Cheeburger has some of the best burgers and milkshakes around. For breakfast, Overeasy Cafe has great food and a really cute outdoor eating area.
If you want to order in one night, Island Pizza is amaaaaaaazing.
Island Cow is always super packed/busy, but my family has found it absolutely filthy (ummm, can you say dried, squished, and smeared bloody mosquito on the wall right next to me during my entire meal; unwiped tables) and will never go back. I get that they're always busy, but the uncleanliness was a real turnoff.
DH and my daughter like Lazy Flamingo--I just think it's alright.
If you can make the trip to Captiva, you absolutely HAVE to pick-up a dessert from The Bubble Room. We make the drive multiple times each trip for their Red Velvet and Orange Crunch cake.