Don't assume they will see you. Be sober - leave the alcohol at home. Wind, sun, vibration and wave motion works with alcohol to increase impairment - in other words, it doesn't take much to be intoxicated. Intoxicated boat operators face stiff penalties if caught. Be seen - make sure your boat has approved lights if you plan to be out after dark.
Most new boats come with lights.
Inexpensive clip-on lights are available at boat and sport stores. In a pinch, make sure a good flashlight is aboard. Be conservative - know your boating skills and don't exceed them. Avoid rough water, difficult rapids or other hazards unless you know you can safely navigate it. Be cautious - even when not on your boat.