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.