Alright, ignoring what I wrote earlier in GM Notes, I have played a bit more with her and she's very fun to play. It would seem that her tail isn't physical, but magic, so I can be more forgiving here than I was before. I do have some things that really bother me about her swimming, though.
She can only go into mermaid form while under water, not when on the surface of it. She blinks in and out of it. She has a bit more oxygen, but both that and her stamina drain really quickly. Honestly... it is very underwhelming for a mermaid.

If nothing else, at least give her infinite stamina when swimming, at all times (so under water and on the surface of it); Hashashin has infinite stamina to run with in the desert. With Corsair, you swim maybe 100 - 200 meters or so and you're all out of stamina, stranded in the water until your stamina regenerates, slow as heck just like with any other character.
For a mermaid, that is really, really silly.

Aside from infinite stamina while swimming, I'd like to see infinite oxygen, but I can let it slide if that doesn't happen I guess. She doesn't transform into an actual fish woman after all. I hope though, with her awakening, since I'm expecting her to fully 'awaken' to the mermaid form she may not be used to yet, that she'll remain in mermaid form while swimming. Blinking in and out of it is too clunky for a mermaid who's gotten used to her powers imo.
I also hope that her awakening will have a Trident, but I guess that remains to be seen.

Finally, her customization - the new long hairstyle is the one I use, but it is once again a hairstyle you cannot edit. What is the point of letting us edit only a handful of hairstyles? Even more important: everyone's mermaid form is the same. Please allow us to customize at the very least the colors of the scales and hair, even if not the magic tail.

In summary:
- She should have infinite stamina while swimming. (That is the most important thing for me and Hashashin has the same for running in the desert, so I don't see why this can't happen)
- Sea Lungs' oxygen should be buffed, preferably with infinite stamina in succession and awakening, I don't mind needing a couple more skill points for that. Preferably it should be infinite, but I can understand it more if this doesn't happen.
- In awakening (and perhaps succession), it would be preferable if she can swim in mermaid form without constantly blinking in and out of it. It's quite jarring, but I can understand it for a mermaid who is not used to her powers yet.
- Corsair should be able to take mermaid form on the water's surface when swimming normally, not only under water.
- New hairs should be customizable.
- We should be able to at the very least give her mermaid form hair and scales/golden 'outfit' different colors. Everyone's mermaid form looks the same.

Here's hoping that Awakening Weapon is a Trident.

+1 to the infinite stamina and at least twice the underwater oxygen.

Also the mermaid transformation only works for horizontal swim it will be more useful if it can be used for fast dives and surfacing because if you press shift while diving it activates the horizontal swim ironically making the mermaid the slowest class to dive and surface since you cant use shift durnig that.

I think the stamina thing is so she doesn't compete with boats. Granted she can't swim as fast as one, but being able to swim between islands seems a bit much (especially if you add a swimming costume). If anything, I'd vote for her consuming less stamina than outright making it unlimited.

I do agree that it's stupid you can't customize the new hair options, but they've been doing that for a while so it was expected. Not wanted, but expected nonetheless.

+1 for a trident, it fits the aquatic theme.

And... none of it happened and awakening is anti-mermaid. Sad. At least it's fun to play, but I would have preferred that trident.

Swimming still being so underwhelming is annoying though. 

She would indeed be slower than a good ship and I see no issues with it. Honestly, a mermaid should be able to swim to Vell and back imo. And yeah, even if it'd take longer, I would totally do that, because I love mermaids. And I hate the sailing system. Alas, she still swims less than half a kilometer and she's a sardine on dry land >.<

Nah remove that aspect focus on her pirate also inf stam for swim is to broken corsair is a sailor not a swimmer

She is both. You might not like tha mermaid aspect, but that doesn't change that she is one.

Why exactly is inf stamina for swimming broken and moreso than inf stamina for running in the desert? Swimming is much slower than using ships, too. You can't attack from the water, either. Please elaborate. That one sentence doesn't give us any info.

I agree but also dont. personally I dont think that inf stamina makes alot of sense. if anything say 1.5 or 2x. same with holding breath 1.25 or 1.5x base stats. My main thing is though I want to see Mermaid from when you are swimming and also get another buff for movement speed in water for like 1.25 or even 1.5x then sprinting can be even quicker due to her being able to summon a water tail.