Place tapioca in a medium mixing bowl along with the water, cover, and let stand overnight.
Drain water from tapioca. Place the tapioca into a slow cooker along with the milk, heavy cream, and salt. Cook on high for 2 hours, stirring occasionally.
In a small bowl, whisk together the egg yolk and sugar. Temper small amounts of the tapioca into the egg mixture until you have added at least 1 cup. Then add this back into the remaining tapioca in the slow cooker. Add the lemon zest and stir to combine. Cook for an additional 15 minutes, stirring at least once. Transfer the pudding to a bowl and cover the surface with plastic wrap. Allow to cool at room temperature for 1 hour and then place in the refrigerator until thoroughly chilled.