Start by making the caramel sauce.
In a heavy, wide-bottom pan, melt the sugar over medium heat.
Stir the sugar frequently so it does not burn.
Once the sugar has melted into a liquid and all solids have dissolved, cook it for a minute more, stirring constantly, until it is the color of mahogany.
Carefully add the water. The caramel may blister into crunchy skins. Keep stirring until all the solids have melted back down and the mixture becomes thick and syrupy.
Pour the caramel syrup into the bottom of a solid tube pan.
Tilt the pan and work the caramel up the sides about halfway, leaving a caramel coating on the sides of the pan. This will help the custard to release after cooking.
Preheat the oven to 350° F.
Add the sweetened condensed milk, half and half, milk, and eggs into a blender.
Blend just until fully mixed.
Pour the milk and egg mixture into the tube pan, aiming the flow directly over the tube. Pouring the custard mixture over the tube first will help prevent over-mixing of the caramel and the custard, as well as reduce the number of bubbles in the custard after baking.
NOTE: If your tube pan has a hole in the tube, seal it with plastic wrap for this step, and then remove the plastic wrap after pouring.
Place the tube pan in a large, oven-safe dish with raised sides (such as a roasting pan).
Seal the top of the tube pan with tin foil.
Fill the roasting pan with hot water to half the height of the tube pan to create a bain-marie.
Bake on the center rack for 1 hour and 30 minutes.
Lift the tube pan from the bain-marie and carefully remove the tin foil so as not to burn yourself in the trapped steam.
Allow the contents of the pan to cool.
Once cooled, refrigerate for a minimum of 6 hours, preferably overnight.
Before serving, use a long, thin, sharp knife to work the edges of the pan, both on the sides and around the tube. Carefully keep to the edge of the pan. The custard is very soft and will be easy to cut into, which may make it break when flipping.
Place a large serving dish with raised sides upside down and centered on top of the tube pan.
With one hand under the tube pan and one hand over the upside down serving dish, flip the two so that the serving dish is on the bottom and the tube pan on top.
The custard should now be transferred to the serving dish, as well as a large amount of caramel sauce.
Use a spatula to remove any caramel sauce still in the tube pan and pour it over the custard ring.