If you dig, you'll hit water five feet down where I live. I found that out while running a new sewer line a couple of years ago. And that was during a dry spell; I imagine the water table rises considerably in the wet seasons.
Most people around here put the shelter 40-50 feet from the house, half sunk into the ground. So you slog out through the mud and rain to get to it, and it's either filled with wildlife or rain. Then you sit in the dark until you think you might be out of danger, then slog back through the mud back to the house.
FEMA has some decent information and recommendations for above-ground shelters. My wife and I have been talking about building one. Instead of putting it out in the yard, we're thinking of going right off the kitchen door, like a sort of heavy duty mud room in a farmhouse. Make it 4x8 and you'd have enough space for a couple of bunks so you could get some sleep to face work the next morning. Put a door opening from the kitchen into the shelter, and from the shelter out to the (relocated) back porch.
A mud-room style shelter would be available without leaving the house, and it would always be aired out and vermin-free, and you'd have electricity and internet, and the refrigerator and toilet only a short distance away if things looked quiet for a few minutes.
Downside: making sure it didn't fill up with "stuff"...