My Little Hippocampus: Friendship is Aquatic

Sooo... I didn't feel like studying for chemistry today... and then this idea got stuck in my head ._. The poses are referenced from what I think is one of the promotional images.

If you've noticed, most of their cutiemarks are different, going along underwater theme, so here's my reasonings
Twilight: No change, since hers in an abstract representation of magic.
Apple Jack: To my knowledge, there's no marine counterpart to apples and, if there were, her name would change too. So I figured, if they can have rainbow-colored, electrified apples, then they can have blue, water-element apples too.
Rainbow Dash In my headcannon, pegasus-hippocampi are amphibious, using their 'wings' to glide above the water akin to flying fish. So, she got her cutiemark by being the first one brave enough to breech the surface, thus it stays the same (Also couldn't think of a marine equivalent to rainclouds >.> )
Rarity Gems to seashells
Pinkie Pie Jellyfish kinda look like balloons, since actual balloons would not work underwater (Although I'm sure Pinkie could find a way)
Fluttershy Butterflyfish!