Final answer:
The mythically significant brackish body of water in leeward O'ahu, which provided sustenance like water and salt, is Āliapa'akai, also known as Salt Lake.
Step-by-step explanation:
The brackish body of water in leeward O'ahu of mythic importance, as well as a source of sustenance in days past, is Āliapa'akai. In Hawaiian culture, bodies of water and land divisions called ahupua'a were integral to both spirituality and daily life.
Each ahupua'a, which extended from the mountains to the sea, provided the necessary resources for its inhabitants. Āliapa'akai, also known as the Salt Lake, was one such resource, providing both water and salt to the local community. Salt was crucial for food preservation and seasoning, and the lake's name itself literally means "salt pond" in Hawaiian.