Cooking salt is better (maybe) when it has impurities in it. Minerals, other salts, volcanic ash, dirt, whatever. It is what gives the salt flavor. When cooking, the shape of the grains also makes a difference: popcorn salt, finishing salt, flaked salt, kosher salt, etc.
In a pool, on the other hand, you want the most boring salt possible. As close to 100% pure NaCl as possible, and ideally as fine as possible. Any additives are at best wasting money, and at worst detrimental.
The problem is, most salt is harvested from natural sources - either from a mine, or from brine evaporation. Contamination does happen. Laboratory grade NaCl is way too expensive for a pool.
Your best bet is to make sure your salt has no purposely added additives - no "free flowing" or "anti caking agents" no iodine. It is kind of like buying bleach - non scented, not splashless, etc. Keep an eye on this site or just search for contaminated pool salt. Look for reputable articles that say certain batches of certain products are causing problems, not crazy "it causes death" type stuff.
Short of that, there is not much you can do unless you somehow plan on testing your salt for purity yourself.
