Yup, that's what I was talking about in my post above....
If you clear too early or too late you end up with a tube filled with water.
Can a purge valve snorkel clear with a passive exhale when on the surface? If so that could even be better?
Also in a rough or choppy sea a freak wave can fill the snorkel without warning. If this happens on the first inhale the you have a serious problem.
I tried a purge snorkel but found it didn't clear fully enough reliably for my needs but I started with simple J so probably biased that way. I think Daniel Man and some American members prefer them. Perhaps better for choppy conditions, especially as the are often paired up with a dry or semi-dry top.
For rough conditions a dry or semi-dry top helps. But I wonder if that is partly because most modern snorkels are very short, too short IMHO. An extra inch and a half of snorkel length improves things (I extended one of my snorkels). A purge valve may or may not.