Elevating a water heater to avoid damage during flooding
In Dallas many houses have water heaters in the attic. This frees up usable space in the living areas, but I have heard from someone in water damage remediation of virtually total loss of a demi-mansion due to the simultaneous failure of multiple  electric water heaters while the family was on vacation.

We have lived in our single story tract house for 40 years and had two different gas fired tank water heaters fail. This caused significant damage even though we were home both times. Our heater is not in the attic but in a service closet on the living floor in the middle of the house.

Thirteen years ago I changed to a tankless heater and feel much more comfortable. I put the tankless heater in the same location as the old tank and if it would leak catastrophically it would cause some damage, but the service life of these tankless heaters is much longer than a tank, and I think the tankless give more warning of impending failure.

In your situation you could consider putting a tankless heater on an outside wall of the outside of the house near where the current tank is. These outside tankless heaters do not need a flue through the attic and roof and if they leak the water runs down the outside wall.

One limitation of modern tankless gas fired heaters is that they require 120 V AC power to operate the controls (and a small heater on the outside models) so if the line power is out, you have no hot water (unless you have a backup generator). We have a first generation Bosch tankless (which has a standing pilot light--yikes!), but it does not require line power so we would have hot water even if the power is out. We have had winter outages of some extent in Dallas--ice storms bring down power lines, but when we replace our existing heater it will be with a modern tankless requiring line power.

EDIT

The major drawbacks of a tankless water heater (besides cost, they cost significantly more than a tank) are:


The tankless have a much larger burner (200 kBTU/h or more vs ~ 40 kBTU/h for a 40 gal tank) and this may require a larger gas pipe, although this may be only from a place already in the house, i.e., the existing buried gas line from the meter to the house is probably sufficient.
The tankless will not supply the volume of hot water that a tank will--until the tank is depleted.
The tankless has more complicated controls which cannot be expected to be as robust as the simpler controls for a tank and will cost more to repair.


I recently talked to a builder who built a small subdivision of 40 or so houses. He offered a tankless water heater hung on an outside wall as an option and few if any purchasers took it. He selected it for his own house and he loves it. Unlimited hot water no matter how many house guests.