Allan Stanley, 76, of Nefyn, was snorkelling around the island of Gozo when he was hit on 19 June.
His widow Diane told the inquest in Caernarfon it happened in seconds. She said: "There wasn't enough time for me to shout or do anything."
A conclusion of accidental death was recorded by coroner Dewi Pritchard-Jones.
The inquest heard Mr Stanley was snorkelling in a bay when a rigid inflatable boat started up.
"A person snorkelling is very difficult to see," Mr Pritchard-Jones said.
"At best it's only part of the top of the head that can be seen.
"Whether the person operating the boat looked or made any efforts to see if anyone was in the water, I can't say."
Pathologist Dr Mark Lord told the inquest Mr Stanley suffered head injuries and his death was "likely to have been virtually instantaneous".

The comedian and actor has been married to Malaak Compton-Rock for 19 years.
A statement issued through his lawyer confirmed the split: "Chris Rock has filed for divorce from his wife, Malaak.
"This is a personal matter and Chris requests privacy as he and Malaak work through this process and focus on their family."
The couple have two children together, daughters, Lola Simone, 12, and Zahra Savannah, 10.
Chris Rock once joked during a routine: "Relationships are hard, man. In order for any relationship to work both of you have to be on the same page.
"You both have to have the same focus. And that focus is... it's all about her."
Chris Rock made his name as a stand-up comic in the 1980s.
The 49-year-old then went onto forge a successful TV career before appearing in films like Dr Dolittle and Madagascar.
In his latest film, Top Five, Rock plays a comedian who is persuaded into having his upcoming wedding screened as part of a reality TV show.
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A grandfather from Gwynedd died after being hit by a boat's propeller off the Maltese coast, an inquest has heard.
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Chris Rock has announced that he's separating from his wife.