I take long, hard pulls on the CE3 and it is not uncommon for me to get juice in between the seal and mouth piece. As Awsum said, it is suction and there is usually some juice that also collects around the needle hold but I don't know if that is the reason for the juice in the mouth piece and it more likely came through the air tube. Since you need to inhale the vapor, and the vapor is produced from the heating of the juice and must make it's way to the air tube, there is no way to make it "water sealed". Long, hard pulls can also be the reason why you get juice on the battery post.
However, in my experience; if it leaks when not in use......there are only a few reasons: 1. you filled it incorrectly 2. you took it apart and put it back together incorrectly 3. it is defective and passed quality control (just toss it) or 4. you have some cracks in the tube itself (really this is #3-1). So.....
If it is 1. make sure you fill it upside down and pull the needle out while it is upside down - if this is a "refill", use the needle to pull out the air first, make sure there are no air bubbles in the syringe before you put juice in.
2. The air post needs to seat firmly in, I have found that getting the seal wet before putting it back gets a better fit and be careful not to crack the tube when you pry the battery post or seal out (you can hold it to the light)
3. Once you buy these, you will likely have spare parts. If you have a cracked tube, replace the tube with a spare one that isn't. If your seal is bad, get a new seal. If what you have is a dud, figure out what is broken and put is aside for spare parts.
If it leaks when you use it, you are probably like me and taking harder pulls - hence the leak. I also see a lot of stress cracks near the barrel (battery post side) when being assembled on recent orders. This is what Awsum was explaining. air gets in through the stress cracks and floods the wicks, getting it into the air hole and down to the battery. Try what I mentioned, when it gets below 1/3 or when it starts to leak.....roll the CE3 horizontally until one wick is covered with juice and the other is clear of the juice and that should stop the leaking and/or just re-fill it. If it does it when it is full....it is a dud or one of 3 above.
As Tramd said, CE3s do leak but they are; in my opinion, the best of the options.