What's the Name of That Book??? discussion

I am trying so hard to remember this book. I must have read it in 2002 or 2003, and I think at that time it was new. It was about a young kid who discovered a hidden land which was protected from prying eyes by fog. The fog was produced by creatures which I believe were described as white and fluffy, much like seal pups. When these creatures sensed danger, they could produce a fog which would cover the land (perhaps it was an island) and no one could find it. There was some kind of treasure or valuable asset that was on this island that needed to be protected.
The cover, if I remember right, had a dark stone archway and a child looking out toward a half-covered island.

I can't remember anything else now, but I will add it if I remember! Thank you.

michaela, is this a picture book (about 32-48 pages) or chapter book?

What was the kid doing when s/he discovered the hidden land?

I added some details to the header/ topic title. Feel free to edit it.

The Secret of Platform 13 by Eva Ibbotson includes creatures called mistmakers, which look like seal pups and create mist when they hear music. There's also a hidden island in the story. This might be the cover you're thinking of:

THIS IS IT! Thank you so, so much. Yay!