Answer: The main theme of the poem "Boating at the Lake" is creativity, imagination, and the joy of simple activities.
Explanation: The poem emphasizes the creative response of the speaker and their cousins to a rainy day. They use their imagination to make paper boats and enjoy racing them down rain-formed streams. This focus on imaginative play, finding joy in simple activities, and the camaraderie among the cousins are consistent motifs in the poem.