As a Beano crossword fanatic for years I'm surprised you've found this so difficult. The sea creature referred to is Davy Jones which is synonymous with his locker - when you look into a locker it is a like peering into a hole - which is the key (being a locker) to the whole answer.
Yes, the answer is therefore 'whole'. Quite straightforward really.
I think you should all apologise to Trevor for telling him he had his letters wrong. Well done Trevor, big hug.