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.

VIDEO: https://youtu.be/uHc1Pd1Yla8