For the caramelized sugar syrup: Place the sugar in a medium-size heavy skillet (not iron), set over moderately low heat, and allow to melt and caramelize to a rich golden brown. Do not stir the sugar as it melts but do shake the skillet from time to time. Add the boiling water, stirring briskly to dissolve the caramelized sugar. Simmer uncovered 8 to 10 minutes until the consistency of maple syrup. Reserve 2 tablespoons of the caramelized syrup for the flan; pour the balance into a chilled, well-buttered, shallow fluted 2-quart mold. Set the mold in the freezer while preparing the flan.
For the flan: Preheat the oven to 325 degrees F. Combine the half-and-half, heavy cream, and sugar in a large heavy saucepan; add in the orange zest and bring to a simmer over moderately low heat, stirring now and then; blend in the 2 tablespoons of reserved caramelized syrup. Beat the egg yolks until frothy; blend 1 cup of the hot cream mixture into the yolks, stir back into pan, and heat, stirring constantly for 1 minute, until it lightly coats the back of a spoon. Remove from the heat and mix in the Port. Strain all through a fine sieve, then pour into the prepared mold. Set the mold in a shallow baking pan and pour in enough hot water to come halfway up the mold. Bake uncovered for 1 1/2 hours, or until a toothpick inserted near the center of the flan comes out clean. Remove from the oven and the water bath; cool for 1 hour, then refrigerate for 4 to 5 hours until firm.
To invert the flan, dip the mold quickly in hot water, then turn out on a dessert plate with a turned up rim; the caramel syrup will come cascading down over the flan. Don't worry if the flan cracks, it will still be creamy and luscious. Cut into slim wedges, if possible, or scoop out, and serve.
Special Equipment: Portuguese flan mold or souffle dish.