Sure, there are some local bars around the port city. Probably won't feel that comfortable there. Not particularly dangerous but not the kind of crowd you might be looking for unless it is poor and intoxicated fishermen. The nightlife on Ometepe is to grab your favorite beverage and person/s and get to a place on the beach or anywhere that is clear above and gaze at the imensity of the star-filled sky. Now that's nightlife. Go get some.