Found: kids fantasy (chapters) lake/ocean monster

Looking for:

Chapter book
read in late 90s, early 2000s (no later than 2004)
read in the US
features 2 kids (or more) and a lake or sea monster
patron remembers an intricate design on the cover
was probably a stand alone book (not a series), but we're not sure

It's not The Water Horse, The Boggart and the Monster, Tales of the Haunted Deep, part of the Magic Tree House or Bailey School Kids series

Wild guess: Danny Dunn and the Swamp Monster. But this is part of a series. Another book in the series has an intricate pattern, but not this one.

Possibly Emma Tupper's Diary?

I also thought of The Sea Egg, although I'm not sure it's really a "monster".

Maybe Nightpool by Shirley Rousseau Murphy? Main part of the story is about a dragon and a boy, but I remember a part where the boy was trapped in a drowned ruin and there was a sea monster with multiple heads on long necks- I think it was called a hydra? Though this might have been in the sequel, The Ivory Lyre. There's a girl who joins the boy in his quest and becomes a kind of love interest in the second book.

None of these were it so far! We have one more bit of information about it - it may have had a lengthy title. Not much to go on, but here's hoping we find it!

How about Eleanor Cameron's "The Terrible Churnadryne"?

touchstone: The Terrible Churnadryne

Update - it looks like it might be Island of the Aunts by Eva Ibbotson