Why in boats

Why do people feel compelled to wave to people in boats?

I went to Tigre again yesterday. Mom and I went for lunch. It was consistently pleasant, quieter than it has been. I guess all the local folk are down in Mar del Plata. While enjoying the boat trip to the restaurant people waved from passing boats and the shore. Why do they do that? No one waves at me when I'm walking down Avenida Santa Fe.... what is it about boats that makes people feel that waving is okay?