Swift Swim is rather boring. The bonus speed in the water would fit better as a secondary in my opinion. I like the idea of her having an advantage underwater, though. Maybe if she had something like Grawl's Bastion, but for staying in the sea rather than on land, it could work much better. Normally I would be against this if only some levels and water and this become useless in other places, but because she's free, I think it works better for her to have a special niche in some levels to help other players.
Siren Song has a good purpose since it makes enemies more vulnerable to her massive damage rating, but it still seems a bit plain, mostly because I have a skill idea similar to that but executed much differently.
Soul Sever is really cool. I love how the higher her current HP is, the more healing that she'll get from it, which is contradictory in a way that makes it more balanced, so that you can't save her from death at low health but also not get an OP bonus at higher health in a directly proportional way to her health and her target's health. It does seem somewhat strong for it to deal 570 damage at full health, but I imagine that the cooldown is somewhere along the lines of Sha'tra's Abduction or Angry God, so it's perfect.
Sea Servants is a bit disappointing after reading Soul Sever. It's just like Arivan's skill, which wasn't very useful anyways. This one is a little better since it keeps her healthy to deal more damage with Soul Sever, but at the same time, the heal from that move becomes useless. This might work better if it was a debuff to enemies, such as:
Sea Servants: Summons 1/2/3 nautical nuisances that stun enemies for 2/3/4 seconds and deal up to 40/60/80 damage each before needing to rest for 20/18/16 seconds.
It would keep enemies in place for her attack as well as preventing them from attacking her, serving both purposes at once in a really cool way. Maybe they could even recharge twice as fast when she's under water to augment the environemtal playstyle that she has.
Maelstrom is also really cool. I have no critiques of it; it works in an cool way that still has a gameplay function that's creative enough to make it interesting to use and variable. Bonus points for originality.
Her stats are all fine. She reminds me of a Nivus-Elora mixture, built for sea combat. Her quotes are also very good references, though, I find it hard to picture her voice. I've never written this before, but I usually can imagine a hero's voice from their quotes, but this one has 2 sweet quotes and the rest, cynical, mean quotes.
I'll do the Valkryie after you read this and reply.