What does a buoy with a diamond symbol indicate?

A diamond on a buoy means danger or warning.

If the diamond is open, it's a warning. It may indicate the presence of a rock, dam, wreck or other danger. Usually, the danger present is indicated under the diamond.

If the diamond is crossed, it means you should not enter. It may be a swimming area (which would be a "do not enter" area for boats), rapids, or some other danger.