On the rack in the middle position, place a pizza stone or an upside-down baking sheet. Preheat the oven to 240 °C (475 °F).
On a lightly floured work surface, gently stretch the dough with your fingers until it reaches 25-cm (10-inch) in diameter. Place on parchment paper. Set aside.
In a bowl, combine the sour cream and water.
Spread the sour cream mixture over the dough. Place the salmon slices, forming a cross in the centre of the pizza. Sprinkle with the cheese and garnish with the capers and onion.
With a pizza peel, place the pizza directly on the stone or baking sheet in the oven. Bake the pizza for 8 to 10 minutes or until the crust is golden brown and the cheese has slightly browned. Serve with lemon wedges.