How do I substitute regular soy sauce or low-sodium soy sauce

Yup, I add half water. So if the recipe calls for 2 Tablespoons of low sodium soy sauce, exchange it for 1 Tablespoon regular soy and one Tablespoon water....

Depending on what you are making, I wouldn't add water. I never use low salt soy. You could just use a little less.

I'm pretty sure it doesn't matter.
Low-sodium tastes a bit less salty and may or may not be healthier for you depending on whether or not reducing your salt-intake will benefit you personally. At most, perhaps use a little less, if you think the saltiness will be overpowering.