Incredulity.Here are two pictures of the wavelet immediately in front of the people, taken in the shadow. In the space between pictures it has come in some number of inches (six or eight, at a guess), which seems a lot for a tenth of a second.
Shallow-water waves move at a speed that is equal to the square root of the product of the acceleration of gravity and the water depth.
Water depth 0.1m, g=10m/s², sqrt(0.1×10)=1 m/s = 4" per 0.1s
and I think that's not necessarily true for water running up the beach, but that's to calibrate your sense of speed. Water is fast, yo.