Sea salt is naturally derived, whereas table salt is generally iodized. This means that the element iodine has been chemically added to the salt.
The debate rages on among nutritionists as to whether or not one or the other is a healthier alternative. Where magic is concerned, a witch ought to use the most organic, naturally-derived materials they can ... but are not in any significant way penalized if they don't. If you simply cannot provide sea salt, iodized salt will work just fine.