SOLVED. YA Fantasy. Sea-something. A boy and a girl dumped into the deep end of the magic pool. [s]

[F.]

I am searching for a book that I read as a child. I read it between 1994 and 2000, I think. The cover of my edition was dark and had a golden snake on it. I think that the first letters of the title were "SE" or "SEA".
This was a book kind of in the style of "The Dark Is Rising," where two young characters -- a young man and a young woman -- are suddenly thrown into deep magical problems with an ominous and tremendously powerful antagonist somewhere in the distance. I think that it might have had an alternate world element, a magical realm that was somewhat separate from the normal world. It might have been set in England/by an English author. Please forgive me for being so vague on the details, but it was quite some time ago and I was very young.

[Capn]

Seaward

[F.]

FOUND IT: Seaward by Susan Cooper