Your camera's meter
can be fooled by certain situations, like shooting into the light (as
in the boat images below) or by dark subjects against light backgrounds.
The answer is to bracket your exposures. This usually means taking one
shot at the meter's recommended exposure, then one above and one below
the recommended exposure. Try it at first with one stop over, and one
stop below the reading, then repeat this in different situations. When
you get the hang of what your camera's doing, you can just do one at the correct exposure, then one under or one over, depending on the situation.
If you're shooting Jpegs, you should try going up or down in half-stop increments, as Jpegs have less margin for error.
Shooting RAW you have more leeway for tweaking the exposure later on in processing, and as I always shoot RAW, I go up or down from the recommended exposure in 1-stop increments. Less fuss out in the field, where I want to concentrate on the lighting and composition
Practice will make judging a scene easier and more intuitive, so that
you'll know what adjustments are needed. See also Digital
One exposure, exposed
for the foreground
(sky burned out)
bracket two exposures and blend later