Bring water to a boil in a 1-quart heavy saucepan. Whisk in tapioca and simmer, uncovered, whisking occasionally, until tapioca is mostly opaque, about 15 minutes.

Meanwhile, place cored and sliced strawberries in a bowl with 2 tablespoons sugar and seeds from half of the vanilla bean..

Into the tapioca, whisk in cream, 1/4 cup sugar, and 1/8 teaspoon salt. Simmer, uncovered, whisking occasionally, until tapioca is cooked through, about 3 minutes. Stir in the other half vanilla bean seeds (scraped from the pod). Transfer to a metal bowl and cool completely in an ice bath, stirring frequently, 5 to 10 minutes.

Pulse strawberries in a food processor with until coarsely puréed. Divide among 6 (6-ounce) glasses.

Spoon tapioca over strawberries. Chill until cold, about 15 minutes.