That shark business is wseird! It was dark. Sou wouldn't be able to see them anyway. Pure BS in my opinion. So, yeah, why did he mention/invent it?
Why did he say it?
Some say people with guilty consciences let the truth out in a disguised way. Unconscious guilt.
Or, he was preparing us for bones and clothing to surface near the shark feeding place,,just in case.
Or maybe by design he let this out, sort of a hint.