>>513355179
The horizon is made of the ground meeting the sky at eye level or in this case the water meeting the sky.
The waves/swell is able to make the water go above this optical limit of your perspective and rise into the sky hiding everything behind them.
This wave hiding shit is a process that repeats infinitely into the distance meaning that all you're really able to resolve is a few miles of ocean after which things will appear to disappear from the bottom up until all you see is water.