Long shot.
It's a picture book. The version I read was an a4 sized book, landscape iykwim.
I read it about 16 years ago.
A girl somehow travels to an underwater castle.
I think she magically goes through a goldfish ornament or similar.
She meets a sea god.
At one point she nearly drowns.
I think there's some significance to a necklace or tiara.
She struggles to get back home.
The illustration is quite lifelike, not cartoony.
If anyone can remember it I would be very grateful.