Stating the obvious for most people on this site I am sure.
SIZE matters.
If you have ever ran a dive charter boat [I have] you stand a better chance of seeing it.
Quote from the DAN site:
DSMBs are available in various sizes, shapes, and colors — some are more practical than others. A 4-foot DSMB is sufficient for divers to learn the deployment techniques for training purposes. Outside of a training environment, divers should use at minimum a 6-foot (1.8-meter) DSMB, which provides better visibility to surface support and other boats, especially in unfavorable seas with large swells.
As for colour, I have 3 orange and one yellow, a size you can stand a better chance of seeing.