Most boat plugs have a gasket that helps to create a watertight seal. The amount of torque (or tightness) required to properly seat the plug varies by manufacturer, so it's important to consult your owner's manual. In general, however, you should hand-tighten the plug until it is snug, then give it an extra 1/4 to 1/2 turn with a wrench. Over-tightening can damage the threads or strip the head, so use caution and don't overdo it.