For many years, legends of the great Leviathan have arisen. Many theories have been developed as to what the Leviathan was, like a giant whale to an unknown undersea dinosaur.
I myself like to think the Leviathan as a kind of scaled sea monster or dinosaur, like the Elasmosaurus or Lock Ness monster. If it really existed, the Leviathan could've used one of two different methods of fire breathing. One method could be that, like the Bombardier Beetle, it used a chemical reaction to spout liquid with a high temperature. Another method could've been a chemical reaction to develop a flammable gas over time, then when danger's around exhale it, and grind teeth together like flint and steel to ignite the gas. The gas would also help it float, as it carried huge scales. This idea came from the documentary, The Last Dragon | A Fantasy Made Real, where the dragon used a flammable gas to breathe fire and help it fly through the air (documentary given below). Either way, whoever comes in contact with this creature is begging for a scourch.
Thank you for taking a look at my set.
P.S. – Here is the documentary.