Children's book- a boy & his dog hearing a female spirit out by the ocean

What I remember story-wise...
* a young boy who has a really big dog
* the dog's fur reminds you of a mop and you can't even see his eyes (example: Komondor breed)
* they needed to go out to the ocean, and it was raining heavily, possibly a rainstorm
* there was a ghost or a spirit that was there (?) I'm sure that the spirit was female
* the spirit might be calling out to them and might be related to the boy but, I am not sure here
* the boy and the dog might have gone out into the ocean with a boat... but I'm not sure here either

External details:
* A fictional children's picture book.
* The color palette & the setting was very blue, murky, and gloomy.
* The cover is probably of the lady spirit in a very murky & gloomy way, with the use of the blue color palette.
* The title might have had the name of the lady spirit, or the title might be a question ("Where...?" "Why....?" "*insert question* ?"). The title might involve something like "Missing" or "Forgotten".
* It's out before 2016, I'm pretty sure? It was definitely the type of book that is available in a PDF to download from the internet.