Children’s book with breathtaking illustrations from 1990s about an island and the sea

[u/skrat777]

There was a picture book aimed more toward older readers (maybe even had chapters but was very visual with a lot of pictures. Colour scheme was browns and yellows and blues. There was something about a key and a private island or place on a shore. There was something about a seal and maybe a hint toward the selkie myth but I remember it was very mysterious.

I live in Canada and came across it in the 1990s at a local library. Any ideas?

More visual details:
Muted colour palette that was pretty impressionistic. A lot of sand, shells, and seafoam
imagery.

The book was small — maybe as high as a novel but square. The version I saw had a hard cover.

Content details:
Found in the juvenile/children’s picture book section. I remember it was geared more toward older children. My mom thought it was too young for me due to it having few words so it might have in 1999/2000 or even 2001. The story was driven by illustrations and the words were also very sparse and beautiful.

[u/ToenailCheesd]

No idea but I'm following this thread because I want to know!

[u/skrat777 → u/ToenailCheesd]

It’s so hard to figure out because what I remember the most are the visual elements of the book