I snorkel and do relatively shallow (to 50' or so) free-diving in fairly choppy water 5 or 6 days a week as a part of my daily exercise routine, and I agree with much of the above advice, with some exceptions:
I prefer to use a rigid J snorkel with a purge valve. I leave the top wide open so it will breathe better. If a wave happens to splash in I just blow it right out again, no matter which part of the breathing cycle I'm in. It's a reflex action. Once you learn how to use an open-tube snorkel properly you will have no need for any of those gimmicks like splash-guards, dry valves, etc. I do like the purge valve, though, as I find that it makes clearing the snorkel much easier during long swims in choppy water. I don't really see any downside to having a purge valve, although the "real" freedivers generally won't use them. Extra drag, you know!
I find a rigid snorkel to be incredibly convenient (it has to be a good fit, of course). The mouthpiece stays right where you want it to be all of the time. You don't even have to bite down, and there's very little pressure on your teeth or gums while you're swimming. It's especially handy in waves and rough water. With a flex snorkel you need to be biting down constantly. That's no way to relax and enjoy! I've been using the same snorkel for over 4 years now (that's over 1,000 dives) in all sorts of conditions and there are still no tooth indentations on the mouthpiece.
I use one of those super-cheap US Divers snorkels that they sell at WalMart for around $5.00. (I found mine on the bottom, though). I use it because the curvature fits my face. I yanked off the silly splash-guard and added a short piece of vinyl tubing (bought for a couple of bucks at the hardware store) to replace it. I also have a fancy, expensive backup snorkel, but it's so big and clunky that I prefer to use my original.
At the end of a deeper or longer-duration dive I generally spit out the mouthpiece right before I surface anyway, so at that point it really doesn't matter what kind of a snorkel it is. I just open my mouth, give my head a little shake and it's out. However, on the shorter, shallower dives that don't involve air hunger I just leave the snorkel in, in which case I begin clearing it right before I surface. Works great.
I'd stay away from the dry snorkels, as those things suck (quite literally) when you dive. They're more for beginners who stay on the surface and haven't learned the proper breathing techniques yet. I consider them to be the equivalent of a bicycle with training wheels.
