We go to Cayo Costa mostly for a beach day, as you approach keep really close to shore and follow around to the Gulf side, keeping the Johnson shoals to your right, you'll see the big sand bar to your right. Anywhere along the shore is perfect, watch Murdock point and beyond, Cayo Costas a State park, only accessible by boat, beautiful and good for finding shells, sand dollars, relaxing etc.
Beware with the ICW, the "Red Right Returning" rule changes, don't ask me why, I've asked but nobody can give me an answer. The beauty is that with a jet boat you can cut corners, but only do this if you know you can or you've travelled in that area before, my general rule is at high tide I pretty go any where I like, leaving some boaters bemused
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The best fishing in that area is the Boca Grande inlet between Cayo and Gasparilla, but you'll see all the boats and realise that.
I've been boating this area since 07, and still find quirky inlets and hot spots, I love it and I'm sure you will, enjoy.