Please excuse the rough sketches...
'They never found the body of her friend. They gathered round an empty grave and bowed their heads... but she knew he wasn't in there. And so she did what any little girl would do... She built a whale and went to find him.'
This could set up the underwater section where we'd see her navigate the world beneath, and eventually find her friend etc.
I quite like the idea of having the mother as a narrator, telling the story of her missing child and providing a v/o for the underwater section. After she finds her friend, we cut back to her mother, who stares out over the lake, and we realise the underwater adventure may have been a fantasy created by a grieving parent. Poignant twist? or too depressing? - you decide!
ps. This idea is playing largely off these 2 lines in saintmaker's fantastic text record...
'While you were looking toward the stars, We quietly slipped into the sea'
'You called us missing persons, But no, we're not missing, We're still your sons and daughters.'