Combine the milk, cream, sugar, and salt in a medium pot.
Spilt the vanilla bean down the middle and add to the milk mixture.
Put the pot on the stove over meidum heat until the milk comes to a boil, whisking occasionally.
While the milk is heating pout the water into a medium-sized heatproof bowl. Sprinkle the gelatin over the water, mix, and let sit for at least 2 minutes.
Whisk the hot milk mixture into the gelatin then pour the mixture into ramekins, cover tightly with plastic wrap and refrigerate for at least 3 hours, overnight is better.
To unmold the panna cotta, dip the ramekin in hot water for 10 seconds, run a knife around the inside edge of the ramekin and invert onto a plate. Tap gently until the panna cotta releases. Serve with blueberry sauce or a fruit preserve.