Stability in the ocean is entirely different than in flat-water and it requires a higher level of fitness. The ocean is dynamic and constantly moving in all directions. To maintain stability in the ocean you’ll use muscles that you don't work out in the flat-water; tiring your body much sooner.
Most folks should modify their stance in the ocean. A common mistake we see is a parallel stance that's too wide, shoulder width apart or wider, with feet placement close to the rails. The thought is that a wide stance will give you stability, it doesn't. With your feet closer to the rails, each weight adjustment and shift buries your rail causing you to shift your weight back to the other side for balance, the result is that familiar tippy feeling. Additionally it doesn’t allow you to shift your hips to make tail to nose adjustments.
Narrow your stance, this concentrates your center of gravity in a smaller space. Stagger your feet from parallel to a front and back position, your front foot will be the same one as your surfing stance. Each board has it's own sweet spot so you’ll have to find it through practice. Start with your front foot forward and the toes of your back foot staggered back and in line with the ball of your front foot. This, combined with a narrower overall stance, will help you shift your hips forward and backward to get that tail to nose movement. Bending a bit more while maintaining a looseness in your legs also helps. Don't forget to keep your eyes up.
Play with it in the flat-water and then test it in the ocean, or go straight to the ocean and work it. It takes time but will come.