Mix the ricotta, sugar, juice of both limes and the grated zest of just one well with a whisk.
Soak gelatine in cold water for 5 minutes, then squeeze out well and stir with the hot water. Mix with 2 tablespoons of the mixture, then mix in with the rest of the mixture. Keep in a cool place until it is slightly firm on the edge, stir until smooth.
Whip the cream until stiff and mix in, pour into four cold-rinsed molds, cover and refrigerate for at least 4 hours.