Calc The Witcher - Caranthir freezes the ocean

Caranthir casts a spell that freezes over a portion of the sea. To stop it, Caranthir must be defeated and his staff destroyed. The sorceresses manage to stop it's spread.
So let's begin with the thickness of the ice sheet. Ciri is 5 foot 9 inches tall (or 1.7526m) at the age of 16 (which would likely make her taller here, but that should serve as a minimum).

The Tower of the Swallow said:

The swordsmith Esterhazy scrutinised Ciri with curiosity, without getting up from the oaken table.

‘It’s for her,’ he guessed, tapping a pen holder against the paperweight depicting the weird group. ‘The sword you requested in the letter. Right, Bonhart? Well, let’s examine it then… Let’s see if it agrees with what you wrote. Five feet, nine inches in height… And such she is. One hundred and twelve pounds in weight… Well, we’d have given her less than a hundred and twelve, but that’s a minor detail. A hand, you wrote, which a number five glove would fit… Show me your hand, honourable maiden. Well, and that agrees, too.’

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459 pixels = 1.7526m
1 pixel = 1.7526m/459 = 0.00381830065m
0.00381830065m X 614 = 2.3444366m
277 pixel = 2.3444366m
1 pixel = 2.3444366m/277 = 0.00846367004m
0.00846367004m X 495 = 4.18951667m

That's the thickness of the ice. We can also see that the ice goes down much, much deeper than that in places.
With that done, let's get the radius. Gameplay wise, the frozen area is over 134 feet in radius (or 40.8432m).
Volume as a cylinder.

V = πr^2h
= π X 40.8432^2 X 4.18951667
= 21956.0048m^3

Weight of ice is 919kg per m^3.

M = 21956.0048 X 919
= 20177568.4kg

Specific heat of water is 4.186 joules per gram. Skellige is a combination of Skandinavia and the British Isles, but given it's geological location, the sea waters of Britain would fit better.
https://www.thebeachguide.co.uk/sea-temperature#:~:text=Overall, the average sea temperature,on region and yearly variation.

Overall, the average sea temperature in the British Isles ranges from 6-10 °C in the winter to 15-20 °C in the summer depending on region and yearly variation.

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Let's find the average.

A = 6 + 10 + 15 + 20
= 51/4
= 12.75

E = 20177568.4 X 4.186 X 12.75
= 1.07690709e9 joules

That's not all, we also have the heat fusion of water to add, that being 334 kilojoules per kg.

E = 20177568.4 X 334
= 6.73930785e9 kilojoules
= 6.73930785e12 joules

Let's add both of those up for our final result.

E = 1.07690709e9 + 6.73930785e12
= 6.74038476e12 joules
= 1.610990621414914 kilotons

Impressive, but there's an issue with this; this is by gameplay mechanics. Chances are through the lore it's much higher. Nilfgaard is around 2560 miles away from Brokilon (Geralt travelled 60 miles, and then there was 2500 miles to Nilfgaard), or 4119.92064km.

Baptism of Fire said:

‘Twelve days have passed since I set out from Brokilon, during which I’ve travelled around sixty miles. Rumour has it that Ciri’s in Nilfgaard, the capital of the Empire. Which is around two and half thousand miles from here. Simple arithmetic tells me that at this rate I’ll get there in a year and four months. What do you say to that?’

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From what I've gathered from Blood of Elves, Geralt was in Craag An.
4257 pixels = 4119.92064km
1 pixel = 4119.92064km/4257 = 0.96779907km
0.96779907km X 141 = 136.459669km
494 pixels = 136.459669km
1 pixel = 136.459669km/494 = 0.276234148km
0.276234148km X 222 = 61.3239809km
454 pixels = 61.3239809km
1 pixel = 61.3239809km/454 = 0.135074848km (135.074848m)
135.074848m X 81 = 10941.0627m

With all of that done, we can finally get onto the volume, mass & energy once more. Volume once again as a cylinder.

V = πr^2h
= π X 10941.0627^2 X 4.18951667
= 1.57555225e9m^3

M = 1.57555225e9 X 919
= 1447932517750kg

E = 1447932517750 X 4.186 X 12.75
= 7.72783304e13 joules

E = 1447932517750 X 334
= 4.83609461e14 kilojoules
= 4.83609461e17 joules

E = 7.72783304e13 + 4.83609461e17
= 4.83686739e17 joules
= 115.60390511472274966 megatons

Final Results
Caranthir freezes the ocean (low end) = 1.611 kilotons
Caranthir freezes the ocean (high end) = 115.604 megatons