Depends on why you watermark
You should not be so concerned with others swiping your designs. What you should be concerned is how to find clients and getting paid. Don't fall into the trap of thinking you're going to lose money at this stage from copying, because even if it happens there's no real way for you to turn it into money.
Only care about money you can actually make. You can look at all the virtual money you've lost, but all that gets you is a bad feeling and is nonconstructive. You're not going to be very efficient in converting that effort into more money. So it's waste of your resources.
Second, even if you would watermark your work it is still not usable in a direct form. Rather, they would take your idea and reuse it because more than likely they need to alter your message. In these cases a watermark does not help, since there's nothing illegal about doing that.
Illustrators have a slightly different value proposition as their images might be applicable for a wide range of uses. Even so they should not care all that much. You can not protect yourself, really, sharing carries risk of re-purposing.
Using watermark as promotional
Now the watermark is not a terribly good defense for shielding you. Instead it can serve as a link back to you. Watermarks in this capacity would work more like signatures, and indeed you should consider not using a prominent watermark but rather have an easily removable one (e.g "done by XXX contact me at XYZ.org for work" etc.)
This is free promotion. In fact showing your work is free promotion. Treat it as such, not as something others take. Even if they do, don't bother, it's inconsequential. Treat the ones that like your work with respect and don't clutter them with things that degrade your work, these are the ones you want to concentrate on. Do promotional stuff with style and respect.