Tension is caused in the scene in two ways. First, the setting is a source of tension. The scene opens aboard a ship in the midst of a violent storm. It is clear that the ship is in danger of sinking. The master of the ship implores the boatswain to rally the sailors to their work, and while the men do their duty with bravery, the ship nevertheless breaks up. In a sense, the play begins with the tragic event that would close most plays.

Second, tension is caused by the relation between the sailors and boatswain and the passengers, who are the king and his retinue. This can be seen in the interchange between the boatswain and Gonzalo, in which Gonzalo seeks information about the ship's status and the boatswain tells him that he is in the way and to go back to his cabin.

This tension is between "high" and "low," or working class and upper class. The boatswain, in ordering Gonzalo below, is asserting his authority on board not just as a sailor, but as a man. Gonzalo, for his part, is condescending to the boatswain and reminds him to think of his passenger, the king, as a way of asserting that his authority is greater. This class dynamic becomes a major theme of the play.

This sense of tension is primarily created through noise. The stage instructions explain how a "tempestuous noise of thunder and lightning is heard" (act 1, scene 1).

The boatswain commands the mariners,

"Tend to th' Master's whistle."

The boatswain also mentions the wind, another noise present. The noises of wind, lightning, shouting, and whistling create most of the tension in the opening scene. A cry is heard, and the boatswain shouts in response,

"A plague upon this howling! They are louder than the weather or our office."

The loud shouts of the mariners and the noise of the storm create most of the tension in the scene. At the end of the scene, several shouts (at once) are described in the stage directions as "a confused noise within":

"Mercy on us!"—"We split, we split!"—"Farewell, my wife and children!"—"Farewell, brother!"— "We split, we split, we split!"

Again, this passage shows a confusion of noise that adds to the tension. Additionally, the repetition of words and phrases, such as "we split" and "farewell," intensifies the sense of confusion and urgency.

A key question with respect to The Tempest is how much dramatic tension it has at all, given that Prospero knows what is happening and controls events the whole time. The first scene is the only clear exception to this concern, because we have not yet met Prospero. So in some ways, the first scene has to deliver the bulk of the dramatic tension in the play. It does so through repeated references to the fear of death. It also creates the tension of anger on stage as it portrays the class struggle between the sailors who are trying to save the ship from the storm and the frightened noblemen who are taking their fear of death out on the poor sailors. The play shows the strength of fear on both sides, which allows the audience to see how dangerous the storm really is. When the scene ends, an audience new to the play might really think the ship has sunk until the next scene, when Prospero reassures Miranda that there has been "no harm done."

Act One, Scene One of Shakespeare's The Tempest opens with the greatest spectacle one could create on the page or stage: that of an enormous storm which puts all the lives of the characters aboard the ship at immediate stake.

Shakespeare methodically paces this tension in order to allow it to build. At the beginning of this scene, the master of the ship is calling for the mariners to "fall to't, barely, / or we run ourselves aground..." There's still hope here that the ship might make it through the storm, and the crew rushes about, with the boatswain screaming orders ("Down with the topmast," "Lay her a-hold, a-hold," etc.) in between the frantic talk of the noblemen on board.

As the scene progresses, the noblemen are ordered to get out of the way and go below deck, but they are there only briefly before crew members begin to descend with them, screaming, "All lost! to prayers, to prayers! all lost!" The ruckus escalates, and the noblemen realize that they are about to die at sea, with the last moment of the scene (and the culminating point of dramatic tension) containing "[a] confused noise within" as the doomed ship sinks to its grave. 

The storm at sea, with the vivid descriptions of lightning and thunder, combined with the shouts and orders of the ship's officers create the tension in this first act.

Shakespeare had to simulate the effect of a ship in a storm with very little in the way of props. The Mariners would be wearing seafaring-type costumes and, of course, the King and his entourage would be wearing the kind of clothing they would wear at court. There would be a strong contrast between the two kinds of characters. The boatswain would be shouting all his lines at everybody regardless of whether Mariners or noblemen. The tone of his voice would suggest the seriousness of the situation, especially because he is obviously spent much of his life at sea and knows more about it than anybody else. This desperate shouting would be important in creating the impression that he has to make himself heard above the howling storm. There would be offstage sound effects. The stage directions at the beginning of Art 1, Scene 1 read simply:

On a ship at sea. A tempestuous noise of thunder and lightning heard.

Shakespeare himself would probably order the sound effects devices and tell the stage hands what he wanted them to do. Thunder can be represented with a big drum. Lightning might be represented by crashing cymbals or other metal against metal. Shakespeare uses similar sound effects in King Lear when he has the mad King shouting in the storm and at the storm in Act 3, Scene 2 of that play.

Blow, winds, and crack your cheeks! rage! blow!
You cataracts and hurricanoes, spout
Till you have drenched our steeples, drowned the cocks!
You sulphurous and thought-executing fires,
Vaunt-couriers to oak-cleaving thunderbolts,
Singe my white head! And thou, all-shaking thunder,
Smite flat the thick rotundity o' the world!
Crack nature's moulds, all germains spill at once,
That make ingrateful man!

No doubt Shakespeare would instruct the stage hands to make noise at the ends of lines so that all the words could be heard by the entire audience. For example after, "Singe my white head! And thou, all-shaking thunder...", there would be a peal of simulated of thunder. The word "thunder" would be a cue for the men offstage to make noise. The same would be employed intermittently on and offstage in The Tempest.

The actual words spoken by the Boatswain would suggest the serious danger, as would the fear shown by the passengers. Both the landsmen and the Mariners are in fear for their lives. The Boatswain has some very good lines. For example, when Antonio asks, "Where is the Master, Boatswain?" the Boatswain replies:

Do you not hear him? You mar our labour. Keep your
cabins: you do assist the storm.

Antonio means the ship captain, but the Boatswain indicates that the real master is the storm. The Boatswain's bad manners suggest that he doesn't care because he doesn't expect to live much longer anyway.

Through costumes, shouting, body language, offstage sound effects, and much of the dialogue in this short scene, Shakespeare creates an impressive amount of drama and tension. And yet it will all be an illusion. There will be no ship, no sea, no wind, no rain, no clouds, no crashing canvas or rigging falling. Only actors on a bare stage. The play could be staged outdoors in a park in modern times and the audience would still have the illusion that they were watching a scene of great distress in a storm at sea. 

Near the very end of Act 1, Scene 1, the Mariners cry out that all is lost. Up to this point the dialogue has been shouted by single voices, but now all the actors representing Mariners would be urged to shout at once as loudly as possible.

‘Mercy on us!’—‘We split, we split!’—‘Farewell, my
wife and children!’—‘Farewell, brother!’—‘We split, we(60)
split, we split!’

When the scene ends, the audience does not know what actually happened. But their interest will certainly be captured and held for a long while.