The speaker describes a group outing that sounds like a shared adventure: going into the thorns together, making breakfast for thirteen guns, sailing through dark and rain. Each moment is tagged with the same word, "pleasure," and the repetition starts to feel less like joy and more like insistence. It's as if the group, or the speaker, is working hard to say everything is fine.
The ending shifts the tone. A guard arrives to "examine our mirth," which suggests that their good cheer is being watched and judged, and the "most abominable lodging" hints that the conditions were bad all along. Judging by the imagery, the thorns, the guns, the dark and rain, the pleasure may be a kind of cover for hardship or confinement. The title, "Esprit de corps," points to group morale, and the poem seems to ask whether that spirit is real or something people hold onto because they have no better option. The mood is brisk and cheerful on the surface, but wary and a little bleak underneath.