NOAA Moving Wave

These groups of waves, formed by wind, will move in the same
general direction as the winds. As a rule of thumb, the wave
groups will spread about 30 degrees in either direction from the
mean source wind direction. The wave density, at the beach, is a
function of area of waves generated, and the inverse of the
square of the distance from the beach. Larger storms generate
more dense swells at the beach, and further away storms generate
less dense swells at the beach.

The really important things to know about wave groups are that
the wave groups travel at roughly half the wind speed, and the
waves travel twice as fast as the group.

The entire point of predicting swells from far away is finding
winds over the ocean that are blowing at you with an adequate
velocity, and then calculating the arrival time based on wind
speed.

NOAA Moving Wave