By the Sea, in a Cave

Near the the 4th Gym Leader's home, is a small cavern. The Seaside Cavern. In the cave you'll meet one of the Water Gym leaders depending on your version. You can't do anything there except for talk to the gym leader and get a PP up hidden in the cavern. After beating the Elite 4, it will then be flooded. You must have surf and dive to get in. Now you can go deeper into the underwater caves. At the end of the caves you'll find the legendary Water Guardian Pokemon, Kureidon.

Credits:
=Snivy101
~Grimmbound
=Pokemon-Diamond