The speed of a wave is related to its frequency and wavelength by the following equation:

Speed (v) = Frequency (f) x Wavelength (λ)

In this case:

* Frequency (f) = 0.5 Hz

* Wavelength (λ) = M (where M is the unknown wavelength)

Therefore, the speed of the water wave is:

v = 0.5 Hz * M = 0.5M meters per second

So, the speed of the water wave is 0.5M meters per second.