SOLVED. Children's Book (age 8-12) about magical adventures with heavy sea imagery // a boy who can speak "the language of the sea" [s]

[Anneliese]

Hello! I've been looking for this book for a while now, to no avail. I think I read it between the ages of 8-10, so it was likely published in the early 2000s (though it could have been a more recent re-print of an older book -- I simply remember that the copy I owned was shiny and new).

The main characters, if I recall, were a boy and a girl around the age of 11 or 12 (maybe younger?). They were adoptive siblings -- the boy had been found and taken in by the family as a baby. He was able to "speak the language of the sea." He had an odd name, I think. The girl was unusual, too. She was able to preform magic feats, such as turning herself and the people around her invisible. The children were mentored by an old man who lived by the sea -- he taught the girl to control her magic and directed them towards their quest. Adrift in an underground lake, a magic boat waited for the children. It took them to a magic land where the boy found his real family. If I recall, his sister was a princess who had been captured and whom the children needed to save. I'm pretty sure that this book was the first in a series, but I never owned any of the others.

In summation:

Characters: a young boy and girl, one who "speaks the language of the sea" and the other who can preform magic feats.

Setting: A fictional sea-side town.

Plot: A mentor teaches the girl to control and grow her powers. He shows them a magic boat in an underground lake that takes them to a magical land, from which the boy originally came. Here he meets his sister, a princess who has been captured and is in danger.

Themes: The sea (very prevalent), magic, mention of "Atlantian technology" in reference to the magic boat

All the details are fuzzy in my head, and I may be mixing multiple books together.

I've been casually searching for this book for about two years now, but it still eludes me. I'm starting to wonder if I imagined the entire thing!

I'd really appreciate your help in finding this book! Thank you!

[Elias]

I'm going to bump this because it sounds really good.

[Ket]

This doesn't match 100%, but if you are mixing books, this might be one of them: Deep Wizardry?

[Tab]

The Witch Trade by Michael Molloy?

[Anneliese]

I think it was the Witch Trade! I still might be mixing books, but The Witch Trade was definitely ONE of the books I'm thinking of. Thank you so much!!