Expanses of water will reflect the sky, giving added interest to your images.
Reflection, © Fernandodelopey 2011, Canon EOS 30D
However you do not have to specifically use lakes or rivers - focus on puddles in the street after a rainstorm for a similar effect or even wet sand by the sea.
Black & white
Clouds within skies naturally lend themselves to black & white photography. Use the 'My Colors' setting on your camera to achieve this and other similar monochrome effects.
Graphic design, © Dario Bonetto 2011, Canon EOS 40D
An interesting mix of colour and black & white can be achieved with the 'Color Accent' menu. This lets you choose one colour whilst the rest of your image becomes black & white.