First, you should know that it's rare for a horse to be deficient in biotin, a B- vitamin that is made in the horse's gut. However, many horses do benefit from a balanced hoof supplement such as Farrier's Formula. Good nutrition is the first step in building solid hooves. If the product you were using improved your horse's hooves, you should have continued feeding it. It is likely that your horse's cracked hoof is made worse by being in a pasture that is flooded.
Excess moisture can make hooves weak and prone to cracking.
That's why cracks are often seen in the hooves of horses that are bathed daily and in those that are turned out in wet grass at night and then stand in dry sawdust during the day. Your chances of getting rid of the crack will be greatly improved if you keep your horse's hooves consistently dry. It is going to be very difficult, if not impossible to improve your horse's cracked hoof as long as he is in the wet pasture. This means you should remove your horse from the pasture when it is flooded and return him only when the pasture is dry. Also, it might help to apply a hoof sealer to his hooves twice a week to help protect them from excess moisture.