Put the sugar in a large saucepan and pour the water all around.
Add the vanilla bean and seeds.
Cook over moderately high heat, stirring, until the sugar is dissolved; use a wet pastry brush to wash down any crystals from the side of the pan.
Cook without stirring until a deep amber caramel forms.
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.
Serve with ice cream, pound cake or fresh strawberries.