URM06 wave dection

Hey I'm trying to get the URM06 to detect waves however I need to mount the sensor face at 15-30 degrees to the water. At that angle the sensor returns 15m which is the same as it not receive a distance but at straight down it reads fine. I don't get any readings back even though its within the 10m range and the surface is disturbed. What can I do to improve my readings?