Preheat the oven to 400°. In a large, shallow baking dish, arrange the halved bananas, cut side up.
In a medium saucepan, combine the sugar and butter and cook over moderate heat, stirring, until just melted. Raise the heat to moderately high, add the boiling water and boil for 3 minutes. Pour the sauce over the bananas.
Bake the bananas for 15 minutes, or until golden and the caramel is bubbling. Serve the bananas warm with the ice cream and pass the nuts at the table.
The bananas can be prepared through Step 2 and kept at room temperature for 3 hours.