The Ship Of adventure: the adventure Series (Book 6) By Enid Blyton

When Jack wins a cruise around the Greek islands, the children are thrilled: sunshine, dolphins, and hidden coves await! But when a valuable manuscript goes missing and a secret message is found carved into an ancient ruin, the ship's cheerful voyage turns mysterious. Someone an board isn't who they seem. Jack spots a man with a snake tattoo in two different ports and Kiki begins mimicking voices no one else remembers hearing. As they explore forgotten temples and treasure-filled catacombs, the children realize they've stumbled anto something much bigger than a stolen scroll. But when the ship weighs anchor unexpectedly and Lucy-Ann is no longer on board, they know the enemy is closer than they imagined.