I'm just posting this here for convenience. Credit goes to Easy Pete (Dunno who you are on the wiki) for making the calc. A few words might've changed but it's nothing major.
The Hydra breaks an 18 x 18 x 14 area of Iron around it. Around 4536 cubic meters.
The Hydra destroys water so we'll assume vaporization.
Iron weighs about 7.87 grams per cubic centimeter, or 7870 kilograms per cubic meter.
4536 x 7870 = 35698320 kilograms
Vaporization value for iron is 6213627 joules per kilogram, according to the calculations page
6213627 x 35698320 = 2.21816045e14 joules, or 53.01530712237154 kilotons
Town level+
Picture Evidence:
