Navasana/ Boat Position targets the core region of your body. If you experience pain then you should take a break. This can be a position that is easily overdone and can lead to long term damage in the lower-back (lumbar) region. Always practice safely.
Conversely, if you practice this slowly and safely on a regular basis it can help in reducing the propensity of injury in the lower back (lumbar) region. This exercise increases the awareness of the muscles in your core region which helps to maintain a healthy back.
Start off practicing for short durations - 3-4 breaths - once per day. Over time you can safely increase the number of breaths. I recommend that each breath take between 3-4 seconds. You can time yourself to have a better idea of where you sit, and experiment with different durations to find what works best for you. If you ever experience pain or discomfort make sure you stop and seek the advice of a medical professional.