What does a regulatory buoy with a diamond buoy?

A diamond on a buoy means danger or warning.

If the diamond on the regulatory buoy 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 on the regulatory buoy 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.