Kailua-Kona has always been the heart of the Big Island's west shore, an epicenter of culture and activity, and a productive area for Hawaiian residents to live. When King Kamehameha I wanted a home from where he could rule all of the Hawaiian islands, this was it. The Ahu'ena Heiau visible in the small bay just north of the pier was the king's dedication to Lono. Not insignificantly, the Kailua Pier was also the location where the first shipload of Christian missionaries disembarked on April 4, 1820 (after the Hawaiians had fought a battle ending their observance of traditional religion). The pier is dubiously nicknamed the "Plymouth Rock of Hawai'i" for this reason, and it is still a bustling pier for ferries, tours, and trade.
Kona Beach sits in front of the Marriott resort and is a very popular public beach for guests and for public alike. Small, quaint, and picturesque, it benefits from the hotel's attention to landscaping, the heiau in the water, and a rental facility where you can find kayaks and stand-up paddleboards to take out for a few hours. There's no better way to view downtown Kona, or you can opt to head north up the coast to Old Kona Airport Beach Park. The beach itself is ideal for families: Because the water is so protected, there is virtually no surf, and the ocean floor gives away very gradually to allow for gentle wading.
Given the location and the relatively small size of the beach, this is naturally a crowded place. It is best to pay for the parking in the Marriott lot...be sure not to park in the area immediately adjacent to the pier. Restrooms and outside showers are available, though the building is pretty run down. No pets, smoking, or alcohol are allowed here.