When I use my SpinnShelter I use the idea from the Goassamer Gear user tips section.. an idea from a customer in the UK, where it rains all the time. The floor is really simple to make. The instructions are on the site.
Another thing you can do, though it's a bit cumbersome, is to take the two corner tie-outs on each end of a ground sheet, pull them together to form a "U" of the end of the ground sheet, and loosely suspend these ends with cord from the hiking poles. This way you have a makeshift "floating floor" that more or less protects from the running ground water. A bit awkward to get in and out of the shelter with, but better than nothing.
I also use my SpinnShelter as a canopy for my hammock, which as you know isn't affected by ground water. However, there are times when hanging isn't an option. What I'm designing right now is a lightweight hammock only three feet wide, but with a top cover made half from Epic and a lower half made from 1.7 silnylon and held open with hiking pole spreader bars, so that when on the ground the hammock works well as a bivy/ bathtub floor (yes, in the beginning I wrestled with how to stabilize a hammock with single lines at the ends and with spreader bars, but I figured out a way that works very well). I'm even contemplating a single tube inflatable bladder insulated with PolarGuard to be incorporated into the bottom of the hammock so that it will form a lightweight sleeping mat that gets me a little more off the ground and insulate the hammock while suspended. What I hope eventually to do is have a simple shelter system with shaped tarp, hammock/bivy, and integrated air mattress so that I can sleep just about anywhere, among the trees or above treeline, even above rocks and on very steep slopes, as hammockers already enjoy.
There must be a better way to take the good things about tarp camping and hammocking and bring them together better. As it is they still seem to be two different camps.