60s or 70s Murder Movie in Vacation House

Movie was in colour and American.
Group of young meeting spend vacation at house owned by one of the friends.

I remeber people had to take boat to get to the house so maybe island or around a lake. Few couples.

Big house but one by one they get killd. I think one gets pushed down into a water well or something. Think the killer was a female but there may have been a plot twist also. Sorry it sounds so generic

The Old Dark House (1962 version)?

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That sounds a lot like the plot of the 1986 film April Fool’s Day. It has been a lot of years since I saw this, however, so I can’t remember if anyone died in a well. It was an American thriller about a group of young people, vacationing together in a large house on an isolated island. They were picked off one by one.
The film had a significant twist at the end.