Salt will only dry out food over a long period of time, otherwise known as curing. Grilling is such a fast, high heat process, the salt has no chance to dry out the fish(but overcooking it will, shoot for medium or medium well, and you'lle be fine). Salt IS necessary for taste though. Most of what we call taste actually occurs in the nasal passage, the tounge only knows sweet, sour, bitter, salty, and some newly discovered one covering savoriness, called umami. By using salt, you connect the toungue to the sinuses, thereby compeling the flavor circle. Salt is the most important ingredient in cooking.