If you're in Sami, staying near Sami or planning on Visiting the Sami area while in Kefalonia then this place HAS to be on your list of things to do. Its a short 10 minute boat ride into a cave and SO beautiful. It costs under 10 euros to visit and a great short activity while you're out exploring!
The best time of day to visit is at 11am - its right before all the big tour buses arrive and the sun is just perfectly above the cave by the time you get down into it. It costs 8 euros to visit and you'll be in a boat with about 6-7 other people.
Even if the line goes out past the awning - dont worry, it goes pretty fast!