I am not a strong swimmer, so prefer snorkeling in the more shallow waters accessed from shore. But I have done some from a boat. For those, I prefer more protected water, with a reputation of calmer waves. You are on a good path - doing your research.
Specifically, these on a South Pacific cruise?
- Kahului, Hawai'i
- Pago Pago, American Samoa
- Suva, Fiji
- Nouméa, New Caledonia
- Sydney, Australia
I can address Kahului, Maui. Definitely options for snorkeling. Opportunities to catch a boat at Maalaea harbor are good, including stops at Molokini Crater and Turtle Town. If you do the February cruise, you will be in Hawaii with the humpback whales - which can be seen in the above area. The Harbor is about a 15 minute drive from the cruise port.
Try a search for the other ports on Cruise Critic for comments by cruisers from companies that have a longer history at these stops, and Viator or TripAdvisor to get an idea of excursions/ “things to do” in these locations. Or look for scuba/ dive shops in these areas, as they often serve more fly-in guests than cruisers.