What you call happiness is nothing but a tumult in the mind, a tempest of passion, frightful to behold even for the spectator on the shore.
[Ce que vous appelez le bonheur, n'est qu'un tumulte des sens, un orage des passions dont le spectacle est effrayant, même à le regarder du rivage.]
Here, in Letter Fifty-six, the Présidente de Tourvel is explaining her perspective on Valmont's version of happiness: specifically, happiness in love or in passion. The difference she perceives in their perspectives on happiness is yet another proof, in her opinion, of why she and the Vicomte are incompatible.
The Présidente uses a metaphor of weather, a manifestation of nature to explain how love appears to her. Man's lack of control over nature is similar to Tourvel's sense of helplessness in the face of her own passions. The storm far out at sea, terrifying even if you are on the shore, is like love seen at a distance. To be in the boat, or in love, would be unbearable.