  Underwater sites are a harsh environment for augmented reality applications.
Obstacles that must be battled include poor visibility conditions, difficult
navigation, and hard manipulation with devices under water. This chapter
focuses on the problem of localizing a device under water using markers. It
discusses various filters that enhance and improve images recorded under water,
and their impact on marker-based tracking. It presents various combinations of
10 image improving algorithms and 4 marker detecting algorithms, and tests
their performance in real situations. All solutions are designed to run
real-time on mobile devices to provide a solid basis for augmented reality.
Usability of this solution is evaluated on locations in Mediterranean Sea. It
is shown that image improving algorithms with carefully chosen parameters can
reduce the problems with visibility under water and improve the detection of
markers. The best results are obtained with marker detecting algorithms that
are specifically designed for underwater environments.
