Deep one hybrid is a valid choice.
Aquatic elves, like mermaids, sahuagin, deep one pure and other aquatic races suffer from not doing to well outside of the water. There are few water combats in here, unless you guys want to go and beat up schools of sharks of course.
As for what the need is. At this point there is a rogue and a witch, anything that compliments that would help with getting picked.
@Phaia. Charisma 10, but appearance 22. Either she is incredibly timid and shy, or perhaps really abrassive.
And yes Mysterica. I would suggest taking a look at the unchained rogue. The free feats and and getting dex to damage on your favourite weapon really makes up for the... wait, do you actually lose something going unchained?