The regulations for preventing collisions at sea also apply to personal watercraft. This means, inter alia, that you must proceed at a "safe speed" in consideration of foreseeability and traffic density as well as rowers and swimmers.
You have a duty to give way to traffic from the starboard side and you have an unconditional duty to give way to sailboats and rowboats. If you have the duty to give way, you must do so in good time so that the other vessels can see that you are giving way. It is important to show consideration and good seamanship to other vessels and to observe one's duty to give way so as to avoid that a trip to sea ends in an accident.
When the sun sets, a personal watercraft also has a duty to carry lights: Green to starboard, red to port side and white to the back. Read more about the regulations for preventing collisions at sea (in Danish).