Lake Hylia appears in Zelda: Echoes of Wisdom, and while there's nothing plot-important to do here, you're likely to stumble upon it while exploring Hyrule or searching for Jabul Waters. If you want to equip more than one Accessory, you should visit the Great Fairy in Lake Hylia to unlock the ability to do so. You'll also notice the large rift on the nearby island. The Stilled Lake Hylia is an optional rift in Echoes of WIsdom, meaning it's not part of the main/story quests. However, if you're trying to max out Tri's level, you'll need to get rid of it.