Adventure at Black Rock Cave

I bought this book when I was in 3rd grade in school in 1961. It is about a young boy and girl who spend the summer on the coast in Canada, and see mysterious lights at night across the bay at Black Rock Island. They row a boat to the island one day and find two men camping there. The men appear to be modern day pirates and warn the children not to return to the island. Later that summer a freak storm comes up and the two children find themselves and their boat washed up on Black Rock Island again. Read the book to see what happens next. It's been 50 years, but I still remember this childhood book I bought in elementary school.