Behold!

"Look!" or "Listen!" or "Pay attention to what I am about to tell you!"

I will raise up many nations against you like the sea raises its waves

Here the word "nations" refers to their armies. Alternate translation: "I will gather armies from many nations that will beat against you like the waves in the raging sea" (See: rc://en/ta/man/translate/figs-metonymy and rc://en/ta/man/translate/figs-simile)