The Crystal Lagoon is a stunning snorkeling & swimming spot in Comino (Malta). It's a beautiful sheltered bay, with crystal clear turquoise-blue water and a lot of fish.
You can only get there by boat, so it's less crowded than the Blue Lagoon (which is also a pretty spot, but more touristic). We booked a day trip with Sea adventures to get there, but there are a lot of other possibilities for every budget.