The Palace Master of the Water Mist Sect was a False True God. So, she never had the opportunity to wield the key. However, this obviously mean that another individual had taken on the role.
Back then, unlike the joke that Anabella had become, the Water Mist Sect key wielder was worthy of the title. Although the current Palace Master was still slightly better, their Key Wielder was a God and was happened to be their former Grand Elder.
Unfortunately, that Grand Elder never gained the recognition of the Mist Seal, so the Water Mist Sect had been without a Palace Master for a while back then.
This aside, according to rules established by their ancestors, the key could only be passed on in three situations. The first was if the key wielder died, obviously. The second was if the key wielder entered a deep sleep and became an Ancestor. And the last was if this individual became the Palace Master.