What is an explanation of "Watching for Dolphins" by David Constantine?

In the poem "Watching for Dolphins" by David Constantine, a group of passengers on a boat bound for mainland Greece stop whatever else they are doing and head out on deck to attempt to sight dolphins swimming in the water. Their destination is Piraeus, which is a port city near Athens on the Greek mainland, and they are in the midst of the Aegean Sea, which lies between Greece and Turkey. Their port of origin might be somewhere in Turkey, or they may be coming from one of the Greek islands such as Mikonos.

Constantine describes a variety of passengers: lovers, an overweight photographer, and children. What all these disparate people have in common is a desire to see dolphins. When they head for the deck, their personal problems and ambitions dissipate as they all engage in scanning the waters. They wonder whether a sighting is more likely if the sea is calm, if it is choppy with waves, or if there are seagulls present.

All of these people are awaiting a transcendent experience, something extraordinary that will take them beyond the usual limits of what they consider normal. The poet compares their fervent waiting to praying. As an explanation of what they are seeking he uses the word "epiphany," which means an illuminating discovery or realization, or the appearance or manifestation of something supernatural. As they wait, they imagine what a dolphin sighting would be like and how everyone will laugh and point out the dolphins leaping out of the water.

However, the boat reaches the port and is surrounded by mundane tankers. No dolphins will venture into the black polluted waters near shore. The passengers disperse and go back to the concerns of their individual lives.

In this poem, Constantine reminds his readers that although we are all different, we all share a desire to glimpse something transcendent and supernatural that lies beyond the world that we normally see. Sometimes our longing remains unrequited and, like the passengers aboard the boat in the poem, we pray and hope to no avail. However, it's good to keep in mind that all of these passengers are hopeful when they go on deck because often dolphins are sighted in the water. If they do not observe dolphins this time, there is always the chance that their hopes will be fulfilled on some future voyage.

In this poem, the speaker describes how "certain passengers" aboard a ship would, with "no acknowledgement of a common purpose," rise and go to the railings to look for dolphins in the Aegean Sea. Without discussing their intentions with one another or even being aware that they had similar intentions privately, these individuals would "lose / Every other wish."
They would, uniformly, consider signs such as the presence of seagulls or the calmness of the waters, awaiting the "epiphany"—almost as though they were praying—hoping to see the dolphins break the surface. The speaker says that, if they were to see the creatures, "We should have laughed and lifted the children up / Stranger to stranger," as though their strangeness to one another does not matter, as though their differences do not exist.
But, that "climax" never arrives—the prayer is unanswered—and, as the ship approaches the city, people give up hope of seeing any dolphins. That hopefulness now seems almost like a dream, as the speaker says that they "woke, blinking" with their eyes "cast down" in disappointment.
This hopefulness, this simple and sweet desire to see the dolphins, free and playing in the water, seems to signal something fundamentally human about these passengers: they recognize a beauty in something that not everyone does—after all, it is only "certain passengers," the speaker included, who get up to look. Not everyone appreciates these simple beauties in the world, and even when we are disappointed in our hope for them, there is beauty in that hopefulness.

"Watching for Dolphins" is one of the most celebrated and acclaimed poems by David Constantine.
A first reading shows the poem to be about passengers hoping to see dolphins while traveling by boat to Piraeus, a port city in Greece.
However, as the poem progresses, it reveals a deeper message about human hopes and ambitions.
Here is an excerpt from the second stanza which shows the same:

Every other wish. Even the loversTurned their desires on the sea, and a fat manHung with equipment to photograph the occasionStared like a saint, through sad bi-focals; others,Hopeless themselves, looked to the children for theyWould see dolphins if anyone would.

This stanza shows how the hope of seeing the dolphins binds everyone, including the children.
Although the passengers are strangers to each other, their desire to see the dolphins arguably makes them one, which is highlighted well in this excerpt from the fourth stanza:

We could not imagine more prayer, and had they thenOn the waves, on the climax of our longing come

The poet uses words such as "we" and "our," which again highlights how the passengers' hopes and desires bring them together. The poem at this point becomes more spiritual and religious with the use of words like "implore," which is also used in religious language.
In the fifth stanza, the poem includes the element of wishful thinking, as the passengers hope to celebrate the sight of dolphins.
However, as we progress to the end of the poem, we understand that the epiphany does not occur—that the dolphins are not seen, which leads to sheer disappointment. In the last stanza, the word "black" signifies the loss of hope. With their "eyes cast down," the passengers exit the boat, returning to their solitary lives in the city.