Sea-Ponies!
Twilight is desperately looking for a counter-spell.
Spike realizes that his fire breath does not quite work well under water
Sea-pony Applejack is kinda disappointed with the lack of apple trees under water.
Sea-pony-Luna is bigger than Sea-pony-Celestia on account of the moon being a huge influence on large bodies of water…
…and what the others are doing is rather self-explanatory.