The sea makes a cameo appearance in every stanza. In the first, dead folks are rising up from the bottom. In the second, the sea is depicted as a "winding" kind of place that's similar to life's twists and turns. And in the third we get to hear the sea with its waves breaking loudly on seashores. All in all it's a prominent symbol that's getting at life, its crazy turns, and its beauty that we often take for granted.
Line 7: Here the bottom of the sea is used as a symbol to help represent just how far folks can "rise up" after death.
Lines 11-12: The "windings of the sea" are like life's twists and turns. Sometimes we get lost in the confusion, but after death those "lying long shall not die windily," meaning things won't be so confusing anymore.
Line 21: Sometimes we take for granted the beauty of the waves crashing against seashores. So here the sea is representative of the natural beauty that's exclusive to life. After all, there aren't any seashores in the cosmos, right?