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.
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.