Great place for a walk in the Narwianski National Park. However, be prepared to tug the "boat/footbridge" with your bare hands. It's definitely easier with more people. Another issue is when the water level is low, so you might even have to jump down or climb up around 1m. If the water level is too low the footbridge from Waniewo side will be closed. However, you can just exit right before the bridge and go on the field and cross the water close by since it's gonna be low (shouldn't be higher than the knees of an adult).
Apart from that before you enter to start fishing, sightseeing or just chilling you should purchase a ticket for each person and depending on activity. Tickets can be bought in the local shops on both sides of the park, information booth or online at npn-m.eparki.pl/bilet_wstepu/10 Tickets cost: 5 PLN for adult and 2.5 for a child/students