The air pressure sensor seems like the best solution to me. All you have to do is put a small water-proof bag at the end of an air tube and fasten the bag flat to the bottom of the tank. The water pressure will compress the air in the bag and pressurize the tube to the same pressure as the water. The air in the tube stays dry. The inside of the pressure sensor stays dry. Mount the pressure sensor outside the tank and it stays dry, too.
Tom