Rivari
Actually, Rivari is a natural sea lagoon which is a reserve for migrating birds, but part of it is a beach. The outer part of the lagoon has a long and narrow strip of sand and gravel, with some low-growing trees for natural shade. There are no facilities and very few people come here for the beach itself (usually they see it from the sea during a boat trip)
Rivari is very close to Embourios, the only inhabited settlement in northwest Milos (about 2 km) and next to Fatourena beach. The road going to Rivari passes by Achivadolimni, after that you should follow the signs for Embourio. When you reach the old church of Agia Marina, you will see Rivari from the above before you descend down to it.
Photos
highlights: natural sea lagoon, beach in Milos, Cyclades Islands, Greece