Marsalforn is essentially the most popular seaside town in the island of Gozo, and together with places like Xlendi and Hondoq ir-Rummien, the area is very popular with beach goers.
The actual beach in Marsalforn centre is quite small but the area of Marsalforn itself offers very spectacular places where you can swim. The area of Qbajjar, for instance, is an amazing spot both for swimming and for hiking in the colder months. The coastal line close to this area has very clear waters and makes it ideal for snorkeling. Swimming here is normally safe, but make sure that the wind is not hitting the coast from the front. There have been some accidents in the past. Just make sure that the sea looks inviting enough to dive in it.