So he dug a trench from the sea, straight to the mountain. The Seal Sister allowed cold waters to rush through the trench and cool the Hearth-Home. Great plumes of steam rose up. It took three days for the mountain to cool enough for Ornn. In that time, the ocean that fed the river dipped several inches.

Average ocean depth is about 12,100 feet, or 145200 inches

3 Days = 259200 Seconds

Stealing numbers from here

The volume of all of the oceans of the world is about 1,335,000,000 km^3 = 1,335,000,000,000,000,000 m^3

Density of Water = 1000 kg/m^3

Mass = Density * Volume = 1000 * 1335000000000000000 = 1.335e+21 kg

c = Specific Heat Capacity (Specific Heat Capacity of Water is 4186)

ΔΤ = Change in Temperature = Tf* - Ti* = 100 - 20 = 80

L = Latent Heat (vaporization) (In this case it would be 2.264e+6 J/kg)

Q = m*(c*ΔΤ+L) = 1.335e+21 * (4186 * 80 + 2.264e+6) = 3.4695048e+27 Joules

Low End (2 Inches):

2 / 145200 = 0.0000137741

3.4695048e+27 x 0.0000137741 = 4.7789306e+22 Joules

4.7789306e+22 / 259200 = 1.8437232e+17 Joules, 44 Megatons of TNT, City level

Mid End (6 Inches):

6 / 145200 = 0.00004132231

3.4695048e+27 x 0.00004132231 = 1.4336795e+23 Joules

1.4336795e+23 / 259200 = 5.5311709e+17 Joules, 132 Megatons of TNT, Mountain level

High End (12 Inches):

12 / 145200 = 0.00008264462

3.4695048e+27 x 0.00008264462 = 2.8673591e+23 Joules

2.8673591e+23 / 259200 = 1.1062342e+18 Joules, 264 Megatons of TNT, Mountain level

Conclusion

...Huh, Mountain level energy from an actual mountain, aint that somethin? It's lower than most of his feats but it's a decent bit feat considering he not only created the Hearth Home, but he and Volibear's fight destroyed it.