joined by two friends who have never dove and want to get certified.
Would suggest friends get certified before the trip, including their checkout dives. That would allow them to do all dives in Hawaii with you.
Would love to do the manta night dive on Big Island, but generally the diving there seems less favorable for beginners
Have been to 3 of the islands - dove on 2 of them - dives on short Oahu stay got cancelled the morning we were supposed to go.
Between Kauai and the Big Island I would choose the Big Island for beginners although on my first dive in Kauai, I saw a humpback whale about 50 yards away which was the diving highlight on that trip. Not saying Kauai is a bad place for beginners but, of the islands, it wouldn't be my first choice.
On the Big Island I would say that a beginner can and would enjoy diving. I have been able to dive two different parts of the coast. In Kona (southwest), dives weren't particularly hard with dive times averaging just under an hour and depths just over 60 ft. (no AOW needed.) Also have stayed in Waikoloa and dove about 10 minutes away in Puako (northwest.) Dive times averaged just over an hour and depths just under 60 ft. First time diving on a RIB and found the process of the dive shop we used, of entering and exiting the water, to be pretty easy. Visibility was very good both places with mostly calm surface and slight currents.
Although I was able to dive with about 100 spinner dolphins in Kona, topography dominates the diving, I enjoyed the diversity of smaller life (frogfish, wrasse, octopi) farther up the coast.
The Manta Ray dive/snorkel was a highlight of our first trip to the Big Island. We did the snorkel as we had 2 non-divers in the group. It's cheaper to do the snorkel and IMO there would be no difference in what you see - mantas swimming up to you versus mantas was swimming down to you.
If you're staying in Hawaii 2 weeks and diving isn't the main activity, you might consider what other topside activities interest you. Each island offers something a little different. If on Oahu and staying in Honolulu, you can get to most every tourist attraction, such as Pearl Harbor, on hop on/ hop off trolleys. Dive shops will pick you up from your hotel. In Kauai and on the Big Island, you would need a car depending on where you stay as you will likely have to get to a dive shop or marina on your own. Tourist attractions on the Big Island are all around the island - although there are tour companies, having your own vehicle allows you to make your own schedule.
Our traveling friends came over one evening with plans of us going to Maui in late 2021, but somehow it turned into 2 weeks in Cozumel.