It doesn’t need to be above it, on the ground is fine, and probably safer.

I camp with a group that sets up 4 showers and a functioning kitchen(two hot water sinks, etc) for reenactment events. They use a propane version of on-demand water heaters, I think 3 total.

Their set up is pallets for the floor, 2” metal conduit for the framing, sump pits and drain lines to disperse the water, and the heaters are in a custom built, open front box for easy access. Works great, we feed and bathe ~150 people for 2 weeks that way.