Put the sugar in a large saucepan and pour the water all around. Add the vanilla bean and seeds and cook over moderately high heat, stirring, until the sugar is dissolved. Using a wet pastry brush, wash down any crystals from the side of the pan. Cook without stirring until a deep-amber caramel forms, about 5 minutes. Gently swirl the pan to color the caramel evenly.
Remove the pan from the heat and add the cream. When the bubbling subsides, bring the sauce to a boil and cook over moderate heat, stirring, until the hardened caramel is dissolved. Discard the vanilla bean. Stir in the fleur de sel and let cool.