Given that we are dealing with nature, and flood waters on surfaces that may have random levels of depth, coupled with water levels can quickly unexpectedly rise, I'm going to say there is really no safe way to drive through flood waters. BUT obviously if you can GUARANTEE that the water won't reach above the bottom you are "probably" safe. I just don't know given random flooding and driving surfaces that may have unforeseen indentations or sink holes, how that criteria can be guaranteed. The Prius like many cars today, doesn't have a lot of ground clearance. So the margin for error is not too big.
If you ever have had to drive your Prius through a creek flood water of any kind, you'll want to look up the document T-SB-0229-12 dealing with inspection and possible repair (more in this post). It divides the severity into five levels depending on how high the water got: up tire sidewall, no higher than wheel rim axle centerline top of rocker panel / lower door edge lower edge of dash panel top of dash panel with increasingly long lists of things to inspect and repair for each level (also depending on fresh or salt water). -Chap
From what I read, unless you have a very expensive car, #4 and #5 would be written off by insurance. Which is a downside of a very expensive car - they can be badly damaged but still "worth repairing".
My rule of thumb for STANDING WATER is just a few inches. Generally speaking, about the height of the tire sidewall, or enough to clearly see the lines on the road. However COMMA!!! Common sense.....isn't, which is why fire and rescue people have to practice recovering fools from cars that have been swept off of the road by water that is NOT standing. People do not appreciate how little running water it takes to sweep a car off of the road, or how easy it is to go ditch diving at highway speeds just from standing water. My Prius is a work car, and for 5 years now it has served me faithfully and well, but there have been a number of times during inclimate weather that I have had to leave it in the barn and drive my POV, which is a truck. Trucks get swept off of the road almost as easily as cars do by running water.....probably MORE so because truck drivers have an over-inflated sense of invulnerability.....BUT.....my truck car drive through much deeper STANDING water than the Prius would ever be capable of.