It's condensation. As Bob46 said, it occurs when cool air meets warm air. It happens more so on aluminum boats, because aluminum can warm and cool very quickly. As the temperature drops quickly at night the aluminum cools very quickly too...at the same time, the boat cover has trapped warmer air from heating during the day. The aluminum cools much faster than the air trapped by the boat cover...creating condensation! Air movement will help minimize the effect...which is why its more noticeable in the hatches...there is no air moving. In fact, the hatches will trap the warmer air, more than the boat cover.