Samoa has many very beautiful beaches, but not all of them are good for swimming and snorkeling. The lagoons are usually very shallow, so the best time for swimming is at high tide. There might sometimes be strong currents in the lagoons, so be careful. Do not swim without shoes of finns! You can easily hurt your feet on the corals, both when walking and when swimming. Sometimes there might also be strong currents.
In Apia the best beach is Palolo Deep, which is a protected area. The currents are sometimes very strong. But if the sea is calm you can easily swim to a part of the lagoon which is very deep where you can see a lot of fish. Do not swim or snorkle at low tide.
On the southern side of Upolu there are several nice beaches, e.g. "Return to Paradise" Beach, which is beautifu but not very good for swimming, Matareva Beach, Salamumu Beach, Aguana Black Sand Beach, Vaiula Beach and others. In addition many of the resorts are located at nice beaches.
On Savai'i there are good beaches at Fagamalo, e.g. at Le Lagoto Resort, at Manase e.g. at Jane's Beach Fales and Vacation Beach Fales, at Fai'a'ai and many other places.