Affects Version/s: Minecraft 1.13.1
Fix Version/s: None
Environment:Java 1.8.0_51 64bit
SO Windows 10
Processor: Intel Core i7 3770k
Graphic card: Nvidia Gforce 770 GTX
Ram: 20 Gb
When you are in the ocean at a height where entities can not do spawn, they start loading and unloading entities very fast. It causes a pretty serious amount of lag. Reducing the amount of rendering distance can fix the problem for a while.
Replicate: position yourself in the ocean and climb to a considerable height, up to y = 200 for example. See the amount of entities that are generated, the ram consumption, console messages, etc.