Yes seltzer water can help settle an upset stomach and even 7 Up soda can help with an upset stomach as well.
Whenever I have an upset stomach I will drink a bottle or two bottles of seltzer water until the stomach stops being upset.
After drinking a bottle of seltzer water the upset stomach starts feeling better but you should continue drinking seltzer water for a few days until the upset stomach goes away.
Some baking soda in a glass of water also can help with an upset stomach although if you have heart disease then you should not drink any baking soda water otherwise it could cause heart problems.
Some other things that can naturally help relieve an upset stomach are some apple cider vinegar added to a glass of water or some chamomile tea or other types of tea.
You can also apply a heating pad to your stomach to help get rid of the upset stomach and then eat things such as Bananas, Rice, Applesauce and Toast.
This diet is known as the B.R.A.T diet which helps with upset stomachs and allows you to eat so you keep your strength.