Wave equation at Kintra Bay, Islay

I was watching the waves breaking on the beach at Kintra on Islay. I decided to try to use the wave equation. I decided that the rock formation in the middle was 5 metres long. That puts the wavelength at about 20 metres. It took about 5 seconds for the wave to pass the feature so the wave speed was 1 m/s. Frequency = wave speed/wavelength = 0.05 Hz. Time period = 1/f = 20 seconds. It got me thinking about whether the speed of waves on the sea is constant. It turns out that it isn't https://en.wikipedia.org/wiki/Wind_wave I need more data because my rough method hasn't measured up to the known theories.