I went to Hanamaua (can't remember how to spell it) Bay from Waikiki in December. If you're spending any time staying in Waikiki I say it is a must do as it is much closer there. I've snorkelled in a few different countries and although the coral at Hanamaua was VERY plain, the fish were amazing! So many fish of all different sizes and very used to humans in large numbers. I recommend getting there early as I found the water was clearer earlier too.
I didn't snorkel at Paradise Cove so cannot compare to there sorry. I did see a monk seal when walking around the beach route (only for the physically able as lots of uneven rocks).
The rainbow reef is something I had planned to do as I loved snorkelling in the equivalent at Typhoon lagoon at WDW but when I saw they had a nice big window to view from the side I didn't bother. For me swimming with the fish in the wild was the best. With kids I think I would have paid for the rainbow reef though because it would be much easier for them without worrying about ocean currents ect.
