A person sits on a bridge in the rain and does nothing in particular. That is the whole scene, and the title, "Idler," tells us it's deliberate. The speaker isn't waiting for anyone or going anywhere. They are simply sitting, "drinking rain," an image that makes the weather feel less like something to hide from and more like something to enjoy.
The last two lines turn the poem toward the imagination. The speaker is "choosing a new ship / to set forth and talk about." Notice that the ship isn't boarded, only chosen, and the voyage is something to talk about rather than something to do. It suggests a person who loves the idea of adventure and the pleasure of storytelling more than the effort of travel. The bridge, a place between two sides, fits that in-between state. The mood is relaxed, quietly humorous, and a little dreamy. The speaker seems content, even a bit playful about their own laziness.