I snorkel for exercise 6 days a week, some of it in very rough conditions that could easily drown a novice, and I can confirm that dry snorkels are totally inappropriate for rough water swimming. The "dry valve" tends to snap closed without warning, and you can end up sucking vacuum at the most inconvenient times. For most people I would suggest using a semi-dry snorkel such as the Aqualung Impulse II or similar. It breathes well and diverts the majority of the splash water, and it will never suddenly clamp shut like a dry snorkel. Refining your snorkel breathing technique is much better than relying on a dry snorkel to keep the water out.
Another point I would like to make is that a great many commonly available snorkels do not breathe properly. They're fine for snorkelers who swim gently and keep their breathing rates down, but if you need to snorkel hard (i.e. either for exercise or during an emergency situation) and you require some serious air exchange then you will quickly discover that most snorkels restrict the airflow excessively, requiring you to work much harder to push the same amount of air through the snorkel. This is due to a variety of factors such as small tubing diameter, sharp bends in the airpath, or worst of all those poorly-designed antisplash devices that are attached to the tops of so many cheap consumer snorkels. The various deflector plates and slots may guide some of the splash water out of the snorkel, but they also block a considerable amount of airflow.
A simple test of any snorkel that you can perform in the dive shop (with permission of course) is merely to take a few forceful breaths and see how much resistance you feel, especially during the exhalation. For a serious snorkeler there should be very little resistance during the exhalation, although I can tell you that probably 90% of the snorkels on the market will fail that test.
The above may not be quite as relevant to a diver, but I thought I'd give you an experienced snorkeler's perspective.
Another point I would like to make is that a great many commonly available snorkels do not breathe properly. They're fine for snorkelers who swim gently and keep their breathing rates down, but if you need to snorkel hard (i.e. either for exercise or during an emergency situation) and you require some serious air exchange then you will quickly discover that most snorkels restrict the airflow excessively, requiring you to work much harder to push the same amount of air through the snorkel. This is due to a variety of factors such as small tubing diameter, sharp bends in the airpath, or worst of all those poorly-designed antisplash devices that are attached to the tops of so many cheap consumer snorkels. The various deflector plates and slots may guide some of the splash water out of the snorkel, but they also block a considerable amount of airflow.
A simple test of any snorkel that you can perform in the dive shop (with permission of course) is merely to take a few forceful breaths and see how much resistance you feel, especially during the exhalation. For a serious snorkeler there should be very little resistance during the exhalation, although I can tell you that probably 90% of the snorkels on the market will fail that test.
The above may not be quite as relevant to a diver, but I thought I'd give you an experienced snorkeler's perspective.