There are many distractions on the water. As the vessel operator, it is your responsibility to constantly monitor your surroundings, on all boats at all hours. You should assign another person on board to act as a lookout as well.
Make sure no passengers or equipment can impede your line of sight. Scan the bow, starboard, and port sides for boaters, swimmers, flags, and floating debris.
You are required to use every available means, including radar and radio (if equipped), to determine whether there is any risk of collision with another vessel. This is not only common sense, it is the law.