Beyond the Grave by Jude Watson

Amy and Dan Cahill are travelling around the world in the search for the 39 clues. These clues lead to an amazing prize. So far, Amy, Dan, and their au pair Nellie Gomez, have travelled through Paris, Vienna, Salzburg, and Japan. Now, they're heading to Egypt in search of an ancient Ekaterina clue.

Ahh Egypt! The Nile, the pyramids, the weather, the beautiful hotels, the palm trees! Not to mention the disguises, the evil old people, and the betrayals...

Amy and Dan are in Egypt for less than half a day when they spot one of their relatives, a fellow Cahill, Irina Spasky, who's also a Lucian. She's also a competitor in the search for the 39 clues. They go on a wild chase through the Egyptian marketplace, both groups getting lost in the process. The good news: Irina has no idea where they are. The bad news: Amy and Dan have no idea where they are.

When they finally get to their hotel, guess what? They find out that it's home to the Ekaterina stronghold (which is a bad thing, by the way). And even worse? They get trapped there. Of course. On top of that? They find out the clue is underwater. Wonderful. Great. Now what do they do?