The poem opens with a cheerful exclamation about a monk who is "at sea." The phrase works two ways. It can mean he is literally on the water, and it can mean he is confused, out of his depth. An anchorite is traditionally someone who withdraws from the world to live in solitude, so the idea of one drifting on the waves is both funny and a little touching.
The speaker doesn't seem worried about him, though. Instead they invite us to "join with him": share a meal of herring, chant, and take a turn at the oars. The mood is warm, playful, and companionable, as if the speaker is turning a lonely figure into good company. The last image, a "brave cup," is odd and charming. It suggests a small, fragile vessel, perhaps a bowl or chalice, setting out bravely on a huge sea. The poem is only a few lines, but it feels light and adventurous, with a hint of gentle humor about faith, solitude, and being carried off course.