From my understanding they tortured him with Iron Maiden and multiple other things but he didn't die until the kid mermaid ate him. Giovanni was really resilient, even though he was drowned he didn't die. And he accepted dead from the kid mermaid because he knew it wasn't his fault, he wanted to save the kid from too much guilt when he came to his senses so he didn't resist.

Did they kill him with iron maiden before dumping his body to the water when the kid mermaid is hungry af (╯TTД TT)╯╧╧
THAT'S SO CRUEL BRUH HE WAS SO KIND NO WONDER THE MERMAIDS ARE MAD AS HELL. WHAT THE FUCK.