Why did Reynie desperately seek help from across the water?

Reynie desperately sought help from across the water primarily because he recognized that his friends, Kate and Constance, were essential for the critical task at hand at the flag tower. He understood that their unique skills and abilities complemented his own and were crucial in overcoming the challenges posed by the Whisperer. Reynie’s determination to have them join him highlights the importance of teamwork and collaboration in their mission, demonstrating that the strength of their group relied not only on his individual efforts but also on the combined strengths of all members. This need for a collective approach in dire situations is a recurring theme throughout the story, emphasizing the value of friendship and cooperation in problem-solving.