1. Arrange six 75–90 mL (2.5-3 ounce) ramekins in a roasting pan with sides. Do not grease. Preheat the oven to 160ºC (325ºF).
2. Place the water and lime juice in a small saucepan and add the sugar. Bring this to a boil over high heat without stirring, occasionally brushing down the sides of the pot with water, until the sugar turns a light amber colour, about 3 minutes. Remove the pot from the heat and carefully pour the caramelized sugar into each ramekin, dividing equally. Swirl to fully coat the bottom of each ramekin, if needed, and set aside.
1. For the custard, whisk the condensed milk with the egg yolks, lime juice, zest and vanilla until smooth. Divide this evenly between the ramekins. Pour boiling water into the roasting pan, around the ramekins so that the water comes up halfway. Bake for about 15 minutes, until the custards jiggle only a little in the centre when moved. Remove the ramekins from the water bath and place them on a cooling rack to cool before chilling for at least 3 hours.
2. To serve, run a palette knife around the inside edge of the ramekin to loosen the custard. Place a plate over the ramekin and then invert both, tapping the ramekin gently so that the custard releases onto the plate.