Like most of you, I've tried anything and everything under the sun for my 4 kids whenever we go on a trip. You name it, we've done it, including the marker on the arms thing, which btw is not a good idea at all per my kids' pediatrician.
I had heard about Tottoos on this site and other sites. I saw them at a local event here in Philly. They showed me what it looked like and it's the coolest thing. It doesn't come off easily like a sticker would, and it's water resistant too, which was a problem for my kids whenever we'd go to the beach or water park. (Tags and labels I had used either wouldn't stay on, or was a pain in the a$$ to remove and put on their bathing suits, etc.)
I'm a single dad of 4 kids, so keeping them in my sights is a major undertaking on these trips. My ex-wife would always just let them run off, even in stores, and would just try to find them later, which does not sit well with me...
Anyways, I got a basic kit from Tottoos a couple of weeks ago, and used them on my kids last weekend in Washington, DC. It worked out great! Even though we didn't go to a waterpark type place, it did rain pretty hard on our last day, and the tottoos got wet, but stayed on as advertised. My kids all liked having a "tattoo" on them, even my 10 year old. I know, I know, a 10 year old would know my name and number by heart, but "just in case"...anyway, he liked it and wore it without arguing.
I have already placed a re-order, as our next trip is to Disney (4th time!!) in July, and we'll be hitting Blizzard Beach for sure!
If you haven't tried it, I'd suggest you atleast invest $15 for it. It's worked great for me!