The two people viewing the sea are a young woman (maiden) and a sailor. The maiden's perspective is given in the first stanza, while the sailor's is given in the second.

The maiden enjoys looking at the sea. To her, it is as lovely and peaceful as a blue meadow. The sea stimulates her imagination. As she looks out at the waves, she thinks of them as "little froth-people." This could be because the whitecaps look like white hair atop the erect forms. The sound of the surf is soothing to her. She imagines it as the voices of the wave-people singing in chorus. It is safe to assume that she has never had any bad experiences with the ocean.

The sailor has a very different perspective on the sea. He is described as "wrecked." Whether this means the sailor is currently stranded by a shipwreck, in current danger from shipwreck, or whether he had experienced a shipwreck in the past is unclear. To him, the sea is a prison with "dead grey walls." If he is or was cut off from home and civilization by a shipwreck, the sea would isolate him from his life and his loved ones. The expanse of the sea would provoke not pleasant sensations and imaginative reflections, but rather fear and loneliness. This is summed up in the words "superlative in vacancy." Because of the shipwreck, the sailor sees the "grim hatred of nature" written on the sea. One who experiences such terror from an element of nature will find it hard to have positive reactions to it thereafter.

Interestingly, Stephen Crane, the author of this poem, almost died at sea himself in a shipwreck that he later fictionalized in his story "The Open Boat." The sailor's feelings about the sea match what Crane may have personally felt toward the ocean after that harrowing experience.

Bear in mind that Stephen Crane is called the first modern writer. He is considered to have introduced Realism to America. He is grim and ironic in style.

The named characters are the maiden and the sailor. Their viewpoints are very different.

The innocent young woman, perhaps standing on the shore or on the deck of a ship, sees the ocean as beautiful and magical. The playful waves are making relaxing little sounds. To her it is a wonder.

The doomed sailor who is lost at sea views the same waters as barren of life, with no rescue in sight. The mention of walls calls up the image of vast and deadly waves. There is a reason the ocean is called cruel and treacherous.

It is entirely a difference of perspective between danger and safety. The two characters see entirely different attributes of the ocean because of their positions in life. The irony is that they are looking at the same ocean.

The two people in this poem are the maiden and the sailor.

They view the sea very differently because they have such different relationships with it.  The maiden can just come down to the shore and look at the sea and think how pretty it looks.  When it's stormy, she can just forget about the sea.  So, to her, the sea is a fun, beautiful thing.

The sailor has to get his living on the sea.  He has to be out on it in all kinds of weather.  It has probably killed people he knows, and he knows it could kill him.  So, to him, the sea is dangerous and scary.