At first and last light, water isn’t all that reflective, but during midday, the water becomes like a mirror. The more light the water reflects, the harder it becomes for you camera to predict an accurate on-the-water exposure, as the camera will try to expose for the bright water.
To overcome this, follow the “sunny-16” rule. The sunny-16 rule says that, in bright sunlight, a camera’s shutter speed should be set to correspond with the ISO you’re using while the aperture is set at f/16.
Another way to offset bright reflected light from the water’s surface is to use a flash to ensure that your subject is well-lit. In bright sunlight, most flashes won’t go off automatically, but most cameras allow you to override the auto flash function and use the flash when you want.
Adapted from The Kodak Most Basic Book of Digital Nature Photography by Russell Graves (Lark Books, $15)