Measure the water into a small bowl, and sprinkle the gelatin on top. Allow it to bloom for 10 minutes, then set the bowl into a dish of hot water and whisk until the gelatin is dissolved.
Meanwhile, melt the butter in a heavy bottomed saucepan over medium heat. Remove from heat and add the sugar, salt, lemon juice and zest. Whisk together and add the egg yolks. Place the pan over medium-low heat, stirring constantly with a wooden spoon. Cook until the mixture has thickened and coats the back of the wooden spoon, approximately 8-10 minutes. Do not allow the mixture to boil or it will curdle. If it begins to steam, remove it from the heat for about 20 seconds to allow it to cool, then return it to the heat.
Once the curd has thickened, whisk in the gelatin and then immediately pour through a fine mesh strainer into a second bowl. Allow this mixture to cool, stirring occasionally.
Once the lemon curd has cooled, beat the whipping cream to stiff peaks using a stand mixer or a hand-held mixer. Add the cream in thirds, folding it gently into the lemon curd.
Divide the lemon mousse between 4-6 bowls and chill for a minimum of two hours before serving. Garnish with raspberries if desired.