The "crest of the sea" is a poetic way of referring to the highest point of a wave.

Here's a breakdown:

* Crest: The topmost point of a wave.

* Sea: A large body of saltwater.

So, "crest of the sea" means the very top of a wave in the ocean. It's often used in literature or poetry to convey a sense of grandeur, power, and the vastness of the ocean.