Prepare 12 glass containers, ramekins or silicone molds. Each container should be able to hold 1/3 cup of liquid.
Rinse the sweet potatoes under running water and rub the skin clean. Pat dry with paper towels. Set aside.
In a medium pot, add a few inches of water and fit a steamer basket inside. Make sure the water doesn’t touch the steamer. Bring the water to a boil. Place the sweet potato(es) in the steamer. Cover the pot with a lid. Reduce heat to medium, and steam until fork-tender, about 30 – 40 minutes. Once cooked, remove from steamer and set aside to cool.
In another medium pot, bring water to a boil. Add tapioca pearls, stir and cook for 10 minutes. Cover with a lid, remove from heat and set aside for 20 minutes. Drain the tapioca pearls into a strainer and rinse under cold water until cool, stirring occasionally. Drain and transfer to a medium bowl. Add 5 tablespoons sugar and mix well with a spoon. Divide the tapioca pearls into the prepared 12 containers. Set aside.
In a blender or food processor, add coconut milk and milk.
In a small bowl, add the cold drinking water and sprinkle the gelatin over. Mix quickly and set aside for 5 minutes.
Cut the sweet potato(es) into half. Use a spoon to scoop out the flesh. Measure out 225 grams sweet potatoes. Add to the blender with the milk. Blend until smooth. Pour into a small pot. Add the rest of the sugar and cook over medium heat until sugar is melted. Add the gelatin and turn off heat. Stir until gelatin is melted.
Carefully fill each container with the sweet potato mixture while stirring quickly to loosen the tapioca pearls. Transfer the containers to the fridge and chill for at least 4 hours. If using a mold, run a knife between the pudding and mold. Turn the mold over and pop the pudding out carefully. Serve cold.