为何水波在深水的传播速度比在浅水快?

因為水波係深水時的波長較長, 波速= 頻率 x 波長 , 頻率不變,所以水波在深水時速率較高

I presume that you are referring to water ripples, such as those occuring in ripple tanks and which is a kind of viscosity waves, but not sea waves which is a kind of gravity wave that occur in oceans.

In the progagation of ripples, apart from the water particles on the surface are set into vibration, those underneath the suface are also forced to oscillate. Such oscillations are transmitted by these particles down to the bottom of the water. Hence, there is actually interaction, through these vibrating water particles between the surface and the water at the bottom. This would cause influence to the speed of the ripples. The shallower the water, the stronger will be the interaction and the slower will be the ripple speed. Thus, ripples in deep water tends to travel faster than those in shallow water because of less interaction with water at the bottom.

Notice that the shortening of wavelength of the ripples in shallow water is a RESULT of the slower speed, NOT the CAUSE of it.