Kalapaki Beach is a public area, as are all beaches in the Hawaiia Islands (none can be private, by law). The Marriott will charge you for use of their beach chairs and facilities, or you can rent from a vendor located at the left end of the beach (facing towards the hotel). The water is generally protected and therefore not too rough, with paddle boards and hobies the principle sports. Don't plan on snorkeling here.
The beach is a fast walk (less than 10 minutes) from the dock where cruise ships tie up, so if you're coming in by ship, this is a good place to get your feet wet in the ocean. The water is generally about 75 degrees in the winter months, warming up as summer approaches and the warmest in the early fall (up to 85). The incline into deeper water is gentle, but this would not be a place to leave young children unsupervised. There are some rocks at the right end where tidal creatures can be found, and the selection of stuff washed up by the waves can be interesting too. Remember, if you take anything of the islands away from the islands, you never get to return and will have a run of bad luck!
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