There are a couple of ways to reduce lag. One of them being to turn off water reflection.
If you press Esc and click on Options, click on the Video tab and scroll down to Water Reflection. Click on that heading and a tick box should appear. Uncheck the box to turn off water reflection.
Also, you could reduce your view distance, which helps too.
To do that, go to the View Distance heading underneath the Water Reflection heading and drag the slider all the way down to 0.
Hopefully after doing these things your game shouldn't be as laggy. ^^
