Datacenters I've been in that were built with "no raised floors" and flooding in mind do have a "raised floor", but it just happens to be solid concrete. This is very much the same as a warehouse or factory floor. It's built several feet higher than the surrounding parking lot, so that everything is level with loading docks.

If you're talking in-building flooding, not much going on there other than the racks themselves keep the equipment off the floor. All power is ran from overhead. A lot of modern "rack groups" (OCS, TSCIF, etc) have water limited water protection in mind.

Not that I've seen. It's just a concrete slab.