Hello chlamy,
LunarTides is correct in his answer that your refraction input is probably a little high. You can also add more roughness to help break that up. Lastly, it appears that you applied this material to a plane which can also cause issues when looking at your water from certain angles.
Here is a link to a water shader tutorial on our wiki. Hopefully that will help clear up some of those refraction issues.
Let me know if this helps,