There are many ways to do the Boat, but the key takeaway is that you lift your legs and upper body off the floor so that only your bum is on the floor. This forces your core to carry the rest of your body as you balance. Remember that the key to core strength is balance exercises?
So you can do it with your upper body lower to the floor like I have in the top photo, or with your upper body higher like the bottom photo. You can also grab your feet to get yourself up higher, or have your palms behind your head.
Here is another variation.
Cheers Eights & Weights!