Moisture and humidity levels greatly affect corrosion because it speeds up the rate of corrosion. For example, when it rains you bring your bicycle inside simply because the moisture left back by the rain will corrode and rust the metals parts of your bicycles easier.

That depends on what is corroding. Water is a polar solvent and dissolves certain corrosive substances (like salts) very easily, but in the absence of those salts it is a lot less corrosive. Some things, however, including some of the more reactive metals, will react with water directly, since water contains hydrogen and is in an oxidized state.