The poem is built on an uneasy contrast: the easy pleasures of a holiday set against something much darker underneath. The speakers, a couple or a group of travellers, kiss and enjoy themselves, but they "do not tell / how all things end." They know that everything finishes, and they choose silence. The second stanza sharpens this. Professing "great kindness / at a plague house" suggests politeness and good manners performed in front of suffering, perhaps with a sense that the kindness is shallow or part of an act.
In the last lines the speaker turns inward. "That pretend mouth" seems to describe the voice of the self, or the kind of talk the holidaymakers share, and it blurs into "the evening sea," which is both a calm backdrop and a sound that keeps coming and going. The speaker seems uncomfortable, even a little ashamed, at how much of this is pretence.
The mood is quiet, beautiful on the surface, and shadowed by dread. Nothing is said outright, which is exactly the poem's point.