Refraction and amplitude

Another logic can be that wavelength of light decreases in water. In order to maintain its time period, it increases its amplitude. But, in that case, I think the intensity should increase. All these logics are coming from my own thought. Therefore, what's the correct reason?

Here's a pic I have made to explain it:

Thanx...

if intensity increases, shouldn't energy increase too..?? N isn't energy constant....???