The phrase "the mighty breakers of the sea" means basically the same thing as "many waves" and emphasizes how great these waves are. Alternate translation: "Above the crashing of all of the very great waves of the sea" (See: [[rc://en/ta/man/translate/figs-doublet]])
The psalmist speaks of where God lives as if it were high above the earth. Alternate translation: "in heaven" (See: [[rc://en/ta/man/translate/figs-metaphor]])