In the poem Whitman conjures up many stark images on a fierce gale blowing across the ocean. His intense use of language and comparison to other powerful emotions lends itself to the intensity of the poem.
The phrase "savagest trinity" is a quick reference to the wind, waves, and midnight. The three alone can be a fearsome foe, but the combined strength at their most intense, or saveagest, are a uniquely awesome power. It also harkens to the Holy Trinity observed in Christianity where the Father, the Son and Holy Spirit all work together in the religion. The comparison to supremely powerful entities conjures and even more awesome spectacle.
The description of the waves as "milk-white combs careering" continues to paint the picture of an intense storm. The "milk-white combs" refers to the white caps of wind blown water. When viewed from sea level the waves look like combs of white on top of the water. These combs "careering" or moving about in a reckless manner helps with the description of power. The movement gives the white caps a mind of their own almost as if they are intentionally seeking out destruction.