Is gravel necessary for post hole drainage in sandy Florida soil?

You may not need to concern yourself with the gravel if sand is present at the bottom of the hole. It will drain. I imagine the reason for drainage is that if water got into the hollow post and froze, would ruin the post. You may not have a freezing problem there in Florida.

What I would recommend is adding a brick or something else like that so the post bears on that instead of sand. The sand may give way and allow the posts to settle more than you want over time.

Do give consideration for the high winds mentioned in an earlier comment.