Place ice in a large mixing bowl, and place a smaller bowl inside it.
Put the chocolate and water in a sauce pan and heat gently on the stove until the chocolate melts and the water and chocolate are combined.
Pour the chocolate into the small bowl, and whip with an electric beater until thickened. This will take anywhere from 3 to 5 minutes. The trick is to stop beating just as the mixture becomes thick and glossy, like frosting. The minute you stop beating it will firm up, so stop beating as soon as it starts to look thick. Quickly spoon it into serving bowls or cups.
If you beat the mousse a little bit too long, it will have a grainy kind of stiff texture, like in the photo below. It will still taste good, though, so eat it as is, or remelt the chocolate in the saucepan and whip it again.