Try turning your water reflections off along with shadows(?) I could be thinking of a different game with shadows.
Also, you could try changing your rendering systems or finding a less laggy spot?
You can also try lowering your viewing distance. It worked for me in the old FeralHeart.
I'd try changing some of the options to reduce lag, including:
~ Turn off water reflection (helps A LOT)
~ Turn the view distance down to 0
~ Turn off nametags if you can, and I'm not sure if it helps but try reducing the size of nametags as well.
You could also try:
~ Closing other programs on your computer
~ Changing your rendering system
~ Try clicking the little arrow in the chat box and unchecking a few of the chats, like local, movie, etc.
~ If you're in a custom map, trying deleting the weather files to reduce lag. Though you can't really do that if you're in public maps, it still reduces lag in private ones!
Hope this helps! c: