We really enjoyed snorkelling in the lagoon, mostly from the Edgewater resort lagoon which is a marine reserve. Its shallow, but there is loads of fish in the reserve, outside the reserves there was way less marine life. We spent up to 4 hrs in anyone snorkel, one day my total time was 6 hrs and was quite cold even with a full 3mm wetsuit. Rarotonga is certainly on my list of going back to, but this time with my kids and probably more camera gear.