Like the previous Pokémon Ranger games, you have the abilitiy, at certain points of the game, to go into Underwater areas. In Guardian Signs, this happens at the Coral Sea, Canal Ruins & Undersea Cavern. These areas house only Water type Pokémon that live in the water. As such, the Pokémon here are unable to go on land, just as the Pokémon from land cannot be brought underwater.
The Underwater sections, while essentially being the same as the main sections, do have one added twist in this game, the Underwater Chases.
The Underwater Chases are unique in that to play them, it asks you to rotate your DS. Do so as shown and you'll find there is a new way to play. This feature is only playable using the Touch Screen and has you, as the Pokémon Ranger, chasing people and Pokémon into the depths of the sea.
The mechanics are quite simple. You can control your position with the stylus and when the bar by the Ranger is white, you can hold down the stylus in order to get a depth boost to catch up with your opponent. However, there are obstacles to face. Each opponent has a variety of attacks in order to try and stop you. If you are hit by these, you will lose some of your depth and 5 points on your Capture Styler's Energy. These vary depending on your opponent but include Energy Balls, Bubbles and Land Mines.