SOLVED. YA Mystery set on a Maine island with a rowboat called the "Doughnut"; boy and his girlfriend solve mystery involving two mysterious men up to something on the island. Read around 1960s?

[Samantha]

An acquaintance of mine in his 60s or 70s read this Young Adult mystery in his youth. I'm guessing he read it in the 1960s. The story involves a boy with a rowboat that the Maine community where he lives has nicknamed the "Doughnut" because it has a perpetual leak from a hole in the middle of it. The protagonist and his girlfriend discover two mysterious men doing something on a coastal island and solve the mystery.

[Rainbowheart]

Adventure at Black Rock Cave

Something strange was happening at Black Rock Island. Chris had seen a faint light moving around there at night. He and Addie were curious. They went out in the Doughnut, Chris' leaky old rowboat. They explored Black Rock and its cave. They ran into two strange men. Pirates? What were the men hiding?

[Samantha]

Rainbowheart wrote: "Adventure at Black Rock Cave

Something strange was happening at Black Rock Island. Chris had seen a faint light moving around there at night. He and Addie were curious. They went ou..."

Thank you so much! This is it.

[Rainbowheart]

Awesome, glad to help out!