What is the difference between true wind referenced to ground speed and true wind referenced to water speed?

True wind is computed by subtracting the vessel speed from the apparent wind. The vessel speed is either referenced to the water (Speed Through Water (STW)) or the vessel speed is referenced to the ground (Speed Over Ground (SOG)). If the STW is subtracted from the apparent wind, then the computed true wind is said to be relative to water. True wind relative to water is usually used by sailors since they are concerned with how fast they are moving through the water. If the SOG is subtracted from the apparent wind, then the computed true wind is said to be relative to ground. True wind relative to ground is usually used by power boaters since they are generally concerned with how fast they are moving relative to the ground.