What does a buoy with orange diamond and black lettering mean?

They are referred to in the US as Cardinal buoys/Regulatory marks and are used to regulate certain areas. Regulatory buoys are white, with two orange horizontally orange stripes. Between the stripes you'll find either an Orange circle or diamond. The diamond is conveying a warning. It could be a rock, a submerged object, shoal, wreckage or other danger. The black lettering underneath will usually indicate the danger.