The relatives of a young rebellious heir to a fortune (Dean Reed), who spends most of his time on a yacht, try to have him committed to an asylum but are mysteriously killed, one by one, in the family villa. 

Apparently filmed in 1969 but not released until two years later and so far remains lost at sea.

According to Mark920 over at the Gentedirispetto forums…

Directed by Lamberto Benvenuti, the cast includes Dean Reed, Isabella Biagini, Gino Lavagetto and Stefania Careddu. A sea rescue drama in which a wife attempts to have her husband committed to an asylum. The ending is particularly engaging for the violent murder of the main character played by Dean Reed. Filming was carried out at Villa Syracuse (standing in for the psychiatric hospital), the sea of Sorrento, and the yacht Laura Idella owned by Laura Doufur. Local Sorrento extras include Franco Iannuzzi (the boat captain), Peppe Iannuzzi and Antonio Ciampa (the sailors), Umberto De Luca Bossi and Raffaele De Martino (nurses in the asylum) and many more. 

It’s worth noting the subtle differences in the two plot descriptions posted above.

Locandina

Cineromanzo, English translation (work-in-progress)

Quattro Fogli Poster (cult70cinema)

Soggettone (courtesy of cult70cinema)

Fotobuste (courtesy of cult70cinema)

ItaliaTaglia Official Government Censor Certificate