Whale Adventure by Willard Price (Paperback, 1993)
'No use fooling yourself. The sharks got him.'Hal and Roger Hunt are hoping to have a whale of a time when they agree to join the crew of a whaling ship. But as they begin working for the evil Captain Grindle, face dangerous battles with sharks, and then find themselves shipwrecked, the boys realise they're about to have a much bigger adventure than they bargained for.