Captain Sharky. The Mystery of the Sunken City

Close the hatches! Prepare for diving!
Deep on the ocean floor, Captain Sharky and his friend Mikey discover an ancient city that sank due to a volcanic eruption. This city holds untold riches, but during their first dive in the bathyscaphe, which the friends are in, they are attacked by a huge sea serpent, and they fail to bring the treasures up from the seabed (this story is included in the book «Captain Sharky and the Treasures of the Deep Seas»).
Of course, Sharky and Mikey plan to return to the sunken city to retrieve those treasures! They set off on an underwater journey again, this time accompanied by Captain Ramirez, who assures Sharky of his friendship and provides his submarine for the journey. But can this captain, whom many consider a cunning and treacherous pirate, be trusted?