Two seas
Kai is the sea and ʻelua is two, contracted to lua. The name describes a place where two currents, two bays or two fishponds meet. It is descriptive rather than unique, which is exactly why more than one place has it.
The Oʻahu Kailua is the windward town behind Kailua Bay. The Hawaiʻi Island Kailua is on the Kona coast and is usually written Kailua-Kona to keep them apart — the hyphenated form exists for the same reason Kamuela exists, so the postal service can tell two Hawaiian places with one name from each other.
Kawainui
Behind the Oʻahu town lies Kawainui Marsh, the largest remaining wetland in Hawaiʻi. It was a fishpond, then taro land, and it still holds several endangered waterbirds. Much of the town is built on what was once its edge.