Cut rhubarb stalks into ½-inch pieces. Add to a medium skillet with 6 Tbsp. sugar and the water. Cook on medium for about 15 minutes, until rhubarb is tender.
Preheat oven to 325°F. In a large bowl, whisk together egg yolks and ½ cup sugar. In a medium saucepan, heat cream and vanilla seeds over medium-high until bubbles have formed around the edge (a small simmer). Add cream mixture to egg yolks in a slow, steady stream, whisking continuously until incorporated. Return to pan and heat for 1 to 2 more minutes, stirring continuously. Pour custard mixture into a large pitcher.
Divide filling among six ¾-cup ramekins or small ovenproof dishes. Then pour custard evenly over top. Put ramekins in a metal pan with high sides. Carefully pour enough boiling water into the pan to reach halfway up the sides of the ramekins. Loosely cover with foil and bake in middle of oven until just set, 45 to 50 minutes (custard should tremble slightly). Transfer custards to wire rack to cool. Then refrigerate, loosely covered with plastic wrap, for at least 2 hours or up to 2 days.
Just before serving, sprinkle 1 or more Tbsp. of turbinado sugar evenly over top; caramelize with a kitchen torch, moving flame steadily back and forth until sugar is deep golden. (Alternatively, melt the sugar 2 or 3 inches below a preheated broiler.)