Walt, for a fast solution that is "good enough" for the keys in this weather?
Any camping supply store (try campmor.com or rei.com if there's none local) can sell you a portable shower
rig. A loop or square or tubing that a shower curtain hangs down from, and you can tie the frame up to the boom or backstay over the cockpit, so you've got a complete privacy curtain around you. Add the showerhead of your choice (gravity feed from a solar shower bag will give you hot water "free") and you are in business, and you can skip the carpentry and plumbing work below. A fast rinse of the cockpit floor after the gray water drains out, and you're done.
Rigging a shower below, you're going to need a shower pan under it, because gray water in the bilge is going to stink fast. Then you need to build the surround over it...and rinse it down and squeegee or vent to prevent mildew and mold...gonna be a much bigger project.