Mix the water with the salt in a bowl then drop the salmon in. Cover & place in the fridge for 8 hours. Remove from the brine, rinse briefly and wipe dry with kitchen paper. Place on a wire rack on a tray & pop back into the fridge for 2-3 hours to form a pellicle, this is the tacky surface of the fish that is created by the proteins in the meat. This captures the smoke. Put a little oil on the skin of the salmon to stop it sticking to the grill.