When practicing boat pose it is best to start slowly, focusing on the foundation and finding proper engagement in the core muscles.
Boat Pose Level 1
Keep your feet on the ground, hands behind your thighs. Begin by engaging your pelvic floor in and up. Activate your Transverse Abdominus by pulling your hip point towards each other and your ribs towards your hips. Make sure your chest is open, shoulders relaxed. Keeping your feet on the ground act like you are pulling your feet towards your butt. Hold this as long as your can while still breathing.
Boat Pose Level 2
Move to level 2 when you feel like your can float your hands up. Keep everything engaged just like in level 1, you are just adding a challenge by extending your arms. Try not to collapse your chest. If you feel any strain in the low back, take the hands to the legs again like in level 1.
Boat Pose Level 3
Move to level 3 when you feel like you can float your feet off the ground. If you lift the feet up and you start to feel wobbly or lose your engagement in the Transverse Abdominus put your feet back on the ground. You can also do this with one foot off the ground and on foot still on the ground.
Remember core strength is cumulative, meaning you are building strength every time you practice. Even when it is practicing gently. It is not about how far you can go but practicing to gain stability and strength.