Calculation based on DontTalks calculation
That's no ordinary size man on the finger, it's Elfman who is huge. I found that Elfman is 2.3 meters tall.
Picture 1 -
2.3/4.5 = 0.511
0.511 x 30 = 15.333
Green Line = 15.333 meters
Picture 2 -
Green line = 15.333 meters
15.333/26 = 0.589
0.589 x 102 = 60.153 meters
Red line = 60.153 meters
Picture 3 -
Red line = 177px
Red line = 60.153 meters
Blue line = 157px
60.153/177 = 0.3398
0.3398 x 157 = 53.356
Blue line = 53.356 meters
So the height of the foot is 53.356 meters
Picture 4 -
We can see that the front left foot is completely submurged in water. Meaning the depth of Lake Sciliora is 53.356 meters.
Rax calculated a near identical value of 53.2 meters.
I believe that the foot is the best way to determine the depth, because Hiro Mashima obviously had in mind how much of the foot was going to be submurged while drawing. Unlike the wheel later on.
Now for the calculation
~ Based on DontTalks calculation
3859.68 m * 11543.952 m * 53.356m = 2377327836.72738816 m^3
Water density 1000kg/m^3
445559606.5536 m^3*1000kg/m^3 = 2377327836727.38816 kg
Specific heat is about 4178 (J/(kg K)), heating by 81°C:
4178*81*2377327836.72738816 = 804530531849609246.33088 J
Heat of vaporization is 2257000 J/kg
2377327836727.38816 kg * 2257000 J/kg = 5365628927493715077.12 J
804530531849609246.33088 J + 5365628927493715077.12 J = 6170159459343324323.45088 J
Result - 1.474 gigatons of tnt
Solid Small Island Level for Natsu