No, peeing in the ocean is harmless. Human urine is 95% water. The urea in urine combines with ocean water to form ammonium, which acts as food for ocean plant life.
Sometimes, yes. Even if you're a really great swimmer, the undertow and the current in the ocean are very, very strong. It's important not to be arrogant.
This usually happens when your prostate swells, which makes it more difficult to urinate. If you're having trouble urinating after surgery talk to your doctor.
Unless you have shipbuilding experience, you should probably hire somebody to do this for you, there are various regulations and materials needed that a company may have a better and...