They mount up to the heaven - The mariners. That it refers to the seamen, and not to the waves, is apparent from the close of the verse: "their soul is melted."
They go down again to the depths - The word here is different from that used in Psalm 107:24, and rendered "deep," but the idea is essentially the same. It is the sea or ocean considered as "deep;" as bottomless. The idea here is, that they seem to descend into the very depths of the ocean.
Their soul is melted because of trouble - It seems to dissolve; it loses all its vigor; it faints. The word used - מוג mûg - means to melt; to flow down; to soften; and is then applied to the heart or mind that loses its courage or vigor by fear or terror. Exodus 15:15; Joshua 2:9, Joshua 2:24; Nahum 1:5. The "trouble" here referred to is that which arises from fear and danger.

They mount up to the heaven - This is a most natural and striking description of the state of a ship at sea in a storm: when the sea appears to run mountains high, and the vessel seems for a moment to stand on the sharp ridge of one most stupendous, with a valley of a frightful depth between it and a similar mountain, which appears to be flying in the midst of heaven, that it may submerge the hapless bark, when she descends into the valley of death below. This is a sight the most terrific that can be imagined: nor can any man conceive or form an adequate idea of it, who has not himself been at sea in such a storm.
Their soul is melted because of trouble - This is not less expressive than it is descriptive. The action of raising the vessel to the clouds, and precipitating her into the abyss, seems to dissolve the very soul: the whole mind seems to melt away, so that neither feeling, reflection, nor impression remains, nothing but the apprehension of inevitable destruction! When the ship is buffeted between conflicting waves, which threaten either to tear her asunder or crush her together; when she reels to and fro, and staggers like a drunken man, not being able to hold any certain course; when sails and masts are an incumbrance, and the helm of no use; when all hope of safety is taken away; and when the experienced captain, the skillful pilot, and the hardy sailors, cry out, with a voice more terrible than the cry of fire at midnight, We are All lost! we are all Lost! then, indeed, are they at their wit's end; or, as the inimitable original expresses it, וחל חכמתם תתבלע vechol chochmatham tithballa, "and all their skill is swallowed up," - seems to be gulped down by the frightful abyss into which the ship is about to be precipitated. Then, indeed, can the hand of God alone "bring them out of their distresses." Then, a cry to the Almighty (and in such circumstances it is few that can lift up such a cry) is the only means that can be used to save the perishing wreck! Reader, dost thou ask why I paint thus, and from whose authority I describe? I answer: Not from any books describing storms, tempests, and shipwrecks; not from the relations of shipwrecked marines; not from viewing from the shore a tempest at sea, and seeing a vessel beat to pieces, and all its crew, one excepted, perish. Descriptions of this kind I have read, with the shipwrecked mariner I have conversed, the last scene mentioned above I have witnessed: but none of these could give the fearful impressions, the tremendous and soul-melting apprehensions, described above. "Where then have you had them?" I answer, From the great deep. I have been at sea in the storm, and in the circumstances I describe; and, having cried to the Lord in my trouble, I am spared to describe the storm, and recount the tale of his mercy. None but either a man inspired by God, who, in describing, will show things as they are, or one who has been actually in these circumstances, can tell you with what propriety the psalmist speaks, or utter the thousandth part of the dangers and fearful apprehensions of those concerned in a tempest at sea, where all the winds of heaven seem collected to urge an already crazy vessel among the most tremendous rocks upon a lee shore! God save the reader from such circumstances!
When, in the visitation of the winds, He takes the ruffian billows by the top, Curling their monstrous heads, and hanging them, With deafening clamours,on the slippery clouds, That with the hurly death itself awakes! Henry IV.
A storm at sea - the lifting the vessel to the clouds - her sinking into the vast marine valleys - the melting of the soul - and being at their wit's end, are well touched by several of the ancient poets. See particularly Virgil's description of the storm that dispersed the fleet of Aeneas, who was himself not unacquainted with the dangers of the sea: -
Tollimur in coelum curvato gurgite, et idem
Subducta ad manes imos descendimus unda.
Aen. iii., 364.
Now on a towering arch of waves we rise,
Heaved on the bounding billows to the skies.
Then, as the roaring surge retreating fell,
We shoot down headlong to the gates of hell.
Pitt.
Rector in incerto est, nec quid fugiatve, petatve,
Invenit: ambiguis ars stupet ipsa malis.
"The pilot himself is in doubt what danger to shun; or whither to steer for safety he knows not: his skill is nonplussed by the choice of the difficulties before him."
See more in the analysis.

They mount up to the heaven, they go down again to the depths: their soul is (m) melted because of trouble.
(m) Their fear and danger is so great.

They mount up to the heaven,.... The waves which are lifted up by the stormy wind, and the ships which are upon them, and the men in them.
They go down again to the depths: one while they seem to reach the skies, and presently they are down, as it were, in the bottom of the sea, and are threatened to be buried in the midst of it; distress at sea is described in much the same language by Virgil and Ovid (m).
Their soul is melted because of trouble; because of the danger of being cast away; so it was with Jonah's mariner's, and with the disciples in the storm; sea roaring, and men's hearts failing for fear, are joined together in Luke 21:25.
(m) "Tollimur in coelum", Virgil. Aeneid 3. prope finem. "Coelumque aequare videtur pontus". Ovid. Metamorph. l. 12. Fab. 10.