Welcome to Maui and to our marathon, jxn3000. I've been watching this thread because I am always curious what frequent visitors like most, and as someone who lives here, I agree with most of the suggestions.
I know that mauimate's "snorkel til you drop" was meant in good fun, but even though you must be fit to be a marathoner and snorkeling isn't rocket science, it's important to pay attention to basic safety rules. Black Rock in particular has some wicked riptides, and I have seen experienced swimmers struggle trying to beat against them. If you can't make progress from one side to the other, get out and walk to the other side and try the other direction. (Just thread your way through the Sheraton property.) The water around Black Rock is fairly deep, and even though there are usually quite a few people there, it's best not to swim alone. Sadly, a 43-year old man from California drowned there earlier this week.
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And while most return visitors are well aware of Hawaii's love of the aina, or land, we also love our water and the creatures in it. Please do not approach the dolphins, seals, or turtles. If they swim toward you, the polite (legal) thing to do is stop and let them pass. It is also illegal to feed them or touch them. Here is a brief NOAA guide for observing them:
Hawaii Viewing Guidelines: Overview - Office of Protected Resources - NOAA Fisheries . The sea turtles are protected by the 1973 Endangered Species Act, which includes fines up to $20,000.
If you follow all of the advice here, you will be very busy indeed. I'm seeing more serious runners in Ka'anapali every day now. We always walk down and cheer at the finish. Best wishes for a fun stay and a great finish.
E komo mai.