Hello again Cruisefun!
Delighted that you found my tips helpful!
I'm sure you'll love Lahaina - it is definitely worth the trek.
We are not snorkelers so I can't give you any first hand experience of that, but I got into conversation with some other passengers about Molokini and a gentleman sitting nearby said that it was not all that good. His recommedation was somewhere off the Kona coast near Captain Cook. I've had a look in my guide book and it could be Napo'opo'o but, of course, I couldn't swear to that.
I was very taken by the idea of being in the crater of a submerged extinct volcano but we never got round to it. Maybe next time.
The beach we were heading for in Kona was Honokohau, but we never made it. It is just too far to walk! I would definitely recommend taking a taxi and booking them to come back for you. By the time we realised how far it was, we were too far from the jetty so we just got on to the nearest bit of sand we could find. This beach was nice and secluded but the sea was rocky so swimming was not all that safe.
The drive to Waimea Canyon was not very long and very possible to do on one's own. I guess it was about an hour from the port but we were on a guided bus tour and we did stop en route.
Have a look at NCL's organised snorkeling excursions in Kona. I'm sure one of them will be just what you're after. If you're doing the same itinerary as we did (leaving on Saturday night), it may not be possible to snorkel and have time on the beach at Honokohau as you're only there for one day. The ship anchors off the coast and you're taken ashore by tender (life boats). Gosh, this is sure taking me back!
Write again if you have any more questions and do let me know how you enjoyed it.