I saw the Nutri-Seawater at Petco. I wouldn't touch it with a 100 foot pole. It says it has a shelf-life of 12 months and it's all natural sea water with additives. *laff* All natural but we add stuff to it? naw, i wouldn't use it for water changes or to fill an entire tank.
idgy - you don't have a location in your profile so can't tell where you are from. when i originally filled my tank i bought water from the Seattle Aquarium for .05 cents a gallon. Since then I bought an RO/DI unit and I use Oceanic salt for my water changes.
My honest opinion is I would never buy anything like live sand in a bag or natural seawater off a store sheld. You don't know what temperature extremes it's been through as well as how old it is or if it is contaminated in any other way. Also it's not cost effective.