While sand can work, it's safer to do the preslope with a mortar bed...once installed, prior to any other work, it won't move around if you happen to kneel or support yourself with a hand while attaching the liner and the setting bed. The bottom of that recess is concrete, I hope!
This is typically done with a mix of sand:cement (called deck mud) in the ratio of 4-5:1...mixed just wet enough to hold together when you squeeze a handful (sort of like a snowball, but maybe not familiar if you've always lived in S FL!). It should hold together, but not drip, when you've got it right when squeezing a handful. It packs like wet beach sand.
WIth a recessed area, you have a few options...you could make it an curbless shower more easily, if that's of any interest.
I suggest you check out
www.johnbridge.com for help in building your shower. So FL with the common high humidity levels is a GREAT candidate for using some more modern materials to waterproof things. I'd seriously consider using a surface applied waterproofing for the whole shower which means that there is much less that can absorb moisture, and it dries out much faster. A couple of products that can do that are Schluter's Kerdi, and Laticrete's Hydroban sheet membranes. When you use one of those, you need to use a drain specific to them, but if you're not planning on replacing yours (it might be a good idea), Schluter does make a conversion drain so you can leave the bottom part in place. When using one of those membranes, you only need the preslope (a single layer), and then the membrane goes on top of it, then you tile it.