[insert standard disclaimer about lining the waterline, why it's a bad idea, why products are rarely if ever officially classified as being safe for the waterline]
Theoretically, if you were going to line your waterline and you wore contacts, I'd suggest not using Fluidlines because they settle as a mass rather than a flexible coating. They last a little longer because they're harder to wear away from blinking, tearing, etc, but they also wear off in big pieces that can smear or aggravate contacts, or block tear ducts easily.
Look more at a pencil liner, as they're flexible and wear off in gradual layers rather than pieces. Powerpoints or Technakohls are your best bet, because they're a little waxier than the regular pencils. You'll need to refresh them more often, but they won't irritate your contacts.
That's theoretically, mind. Lining the waterline is a bad idea and you shouldn't do it at all, and now no one can say that I gave you bad advice.