The lack of tourists means there isn't a regular dive shop here. Ask at your hotel if anyone is arranging scuba diving. Pacific Underwater Construction has a diving decompression chamber on Tutuila.
Unfortunately, large areas of flat coral around Tutuila have been pulverized by fishermen standing on them or breaking off pieces to extract marinelife. On the upside, the currents aren't bad if the sea is flat, and spearfishing with the help of scuba gear (which decimates fish populations) has been banned.
Snorkeling in the polluted waters off Utulei and Faga'alu is not recommended. The closest points outside the harbor are the open reefs opposite Aveina Bros. Market at Matu'u, or at Lauli'i. These can be treacherous when the southeast trade winds are blowing and there's a rather heavy break. If the water is quiet, get into one of the avas—the channels going out—and enjoy undersea caves and canyons. Just beware of sneak bumper waves and strong currents in the channels: You might have to come back in over the reef, and the break varies considerably. Neither spot is outstanding, however, and you might see more trash than fish.
Better snorkeling locales are found at the east and west ends of Tutuila, but beware of strong currents and undertow. The north coast is best of all, as it's well protected from most pollution and the prevailing winds. However, better snorkeling by far is available at Ofu in the Manu'a Group. Shark attacks are extremely rare around American Samoa—coral cuts and undertow or currents are much more of a hazard.
Local members of the Pago Pago Yacht Club at Utulei come out on Sunday afternoons to take their hobie cat sailboats out for a spin around the harbor. If you befriend someone, you might be invited along.