Mostly any beach in the Mediterranean Sea, some in Southeast Asia, South America and Australia... just kidding. Beaches here look nice in pictures but often there are not that great (mainly because of the sand which is earthy dusty and the extreme heat starting in May.
Here in Doha city, they destroyed all public beaches and all we have left are the hotels (Intercontinental Hotel, perhaps the most popular, not necessarily the best as the beach is artificially prepared the sea floor is construction debris covered with algae.
Recently they have built a series of artificial beaches, between the Intercontinental and the Pearl, in Qatara (a cultural village partially operating).
The beaches are prepared to be public but they are subjected to a cover fee.
All these beaches are artificial so expect dusty sand and sea bed with construction debris.
If you want a natural sandy beach with the desert dunes in the back set, you need to drive to the Sealine (you need a 4x4).
There are a few others spots along the Qatari coast, often the water is too shallow, crowded or no facilities at all.

Desert beaches - no entry fee
Qatar New Artificial Beach - should be a small amount as is meant to be public (20 QR aprox).
Hotels - 100 to 200 QR/day or you may join annual membership 7000 - 25000 QR/year