It would be very difficult to keep a safe water proof. The door would have to have some method of keeping the door extremely tightly pressed against the safe in order to keep that gasket compressed and therefore water tight.
Here’s the theory. It’s the same theory that puts water in a gas tank or moisture in a ammo can.
The inside temp of the gun safe is at house temperature. The gasket is only good at keeping moisture out when the temps inside the safe and outside are about equal.
Once the cold water comes in, the outside temperature drops to water temp. This eventually causes the inside temp to drop, which decreases pressure causing a suction. This suction will draw moisture past the seals. Additionally, the higher the water level, the higher the pressure outside the safe, so the water pressure will actually be pushing water into the safe.
Pelican gun cases inside a gun safe.
The resulting safe would be considerably more expensive however than those currently at your local gun store.
Concentric O-rings for example or inflatable gaskets perhaps. Torque down the bolts on the door to 50 ft. lbs and your safe is rated to 100 ft of water. Proprietary case hardened blot heads would be an added layer of protection against thieves.
With Safe Storage laws a possibility there may be a market for such options in safes on the horizon.
Flex Seal.