I'm not familiar with typical building practices in FL, but filling that area with sand sounds like a bad idea to me. It seems to me you would be creating a nice cavity in which water would tend to sit for a long time, which in turn could cause staining, mildew, moss, etc on all the surrounding surfaces.
Far better to use a very coarse crushed stone, old broken up concrete/bricks, or some other recycled masonry. This would leave lots of air spaces that will encourage moisture to leave the area, as opposed to the sand helping it to hang around.

Your local building codes should define what is required in the way of a footing. I always like to exceed the requirements a little.

Laying up cinderblock & brick is fairly easy for the average handy DIY'er to do is the structure will not be seen.
If it will be visible, pay someone who does this for a living. The result will be dramatically more eye appealing.