Answer: Absolutely, positively NO! Sacrificial anodes, whether they be made from zinc, aluminum or magnesium are part of a system. The system is known to tech types as a "cathodic protection" system. In fact, the ABYC standard E-2, which is entitled Cathodic Protection Systems explains all the requirements for such a system. Anyhow, this system requires both an anode and some cathodes. The anodes, or zincs as our questioner describes them are the sacrificial component. But, it must be submerged in the SAME electrolyte as the cathodic pieces (in this case the through hull fittings in question) it is intending to protect. So, that means the water the boat is floating in. This is all a part of what is known as a galvanic cell, which is comprised of an anode, cathode, electrical connection between the anode and cathode and again, a common electrolyte, in this case the water the boat sits in.