Interested in Language
When we were in the water, we could hear people rushing about on the deck. That's when the panic must have started – when they found there weren't any lifeboats left. I was terrified. I didn't know what a shipwreck would mean or how long it would take but I was too terrified to do anything but shriek for my father I knew we'd left behind.
The general meaning of this bolded fragment means, in my opinion, the following:
Knowing that we had left behind my father, all I could do was shriek for him (father).
Do you agree that there is 'that' or 'who' hidden in the sentence.
I was too terrified to do anything but shriek for my father who/that I knew we'd left behind.
Do you agree with my conclusions?
Yes, "who" is implied/hidden/omitted.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.