Hazards and obstacles
The primary hazard deep water presents is drowning the player. If the player remains underwater for too long, the playable character will start to run out of air, resulting in an instant death once the time is up. To extend one's time underwater, the player needs to collect Air Bubbles. Alternatively, the player can equip an Aqua Shield for an infinite air supply. When being submerged in water, the playable characters' movements also becomes sluggish, although their jumping height will increase. For the characters who can Swim or something similar, however, the player is able to move around everywhere the water reaches.
While some pools of water have bottoms, water may sometimes cover up bottomless pits, meaning the player will first sink into the water before entering said pit. If the playable character is running fast enough however (starting from Sonic the Hedgehog 3), the player will be able to run across water surfaces, allowing them to reach new areas or bypass underwater obstacles.
Attacks and powers
In a few games, Water has been incorporated as a special power or gimmick that the player can use in conjunction with their attacks for more potency.
Sonic the Hedgehog (1991)
Sonic the Hedgehog 2
In Sonic the Hedgehog 2, water appears as an obstacle in Zones such as Aquatic Ruin Zone. The game also introduces Mega Mack, a pink substance that is described as poisonous but still functions like water gameplay-wise.
Sonic the Hedgehog 3 & Knuckles
In Sonic the Hedgehog 3, Sonic & Knuckles and their add-on continuation Sonic the Hedgehog 3 & Knuckles, underwater sections appear in several Zones, including Angel Island Zone and Hydrocity Zone. The game also introduces the Aqua Shield, a variant of the Shield that lets the player breathe underwater. Also, entering the water with the Fire Shield or Lightning Shield will cause the player to lose the shield (with the latter causing the water to flash).
In Sonic Adventure and Sonic Adventure DX: Director's Cut, most bodies of water, such as those found in Emerald Coast, are bottomless pits. However, there are certain underwater sections that the player can safely submerge themselves in, primarily with Big the Cat. With the Life Belt Level Up Item, Big can as well float on water surfaces. In the game's Chao Gardens, there are also swallow water ponds for the Chao to play in.
Sonic Advance series
In Sonic Advance, Sonic Advance 2 and Sonic Advance 3, underwater areas can be found in the various Zones which the player can submerge themselves in or run across. In Sonic Advance 3, by selecting the right character(s), the player can either swim, breathe or float in water.
Sonic Adventure 2
In Sonic Adventure 2 and Sonic Adventure 2: Battle, traversable underwater sections appear in the Hunting Stages. By using Swim, the player move around everywhere while underwater. In addition, Knuckles can obtain the Air Necklace Level Up Item which lets him breathe underwater. Like in Sonic Adventure, the games also include swallow water ponds in the Chao Gardens for the Chao to play in.
Sonic Rush series
In Sonic Rush and Sonic Rush Adventure, underwater areas can be found in the various Zones which the player can submerge themselves in or run across. Some gimmicks also make use of water to help the player traverse the different Acts.
In Sonic Unleashed, water skimming is often a mandatory function in Stages such as Dragon Road, Cool Edge and Jungle Joyride, where the player has to run across waters to reach new areas. In the latter Stages, streams of water can also burst out of the water pools and damage the player.
Sonic Chronicles: The Dark Brotherhood
In Sonic Chronicles: The Dark Brotherhood, the game has an element system common to most fantasy RPGs. Water is one of its elements. Organic enemies, N'rrgal and Voxai are resistant to water while robots, Kron and Zoah are weak against it. Ghise and equipment such as Water Rings add water damage to attacks while Perty grants protection against water.
In the Wii version of Sonic Colors, underwater sections made an return after being absent since Sonic Adventure 2, although they only appear in the game's 2.5D sections. While underwater in this game, the player can continuously Double Jump to ascend. Alternatively, the player can use the Yellow Drill to swim freely around at high speed.
In Sonic Generations, a number of re-imagined underwater sections appear in Chemical Plant, Seaside Hill and Water Palace along with a few water skimming sections in their second Acts. Additionally, in the HD versions, water-skimming actions are present in the Perfect Chaos boss fight.
Sonic Lost World
In Sonic Lost World, the player can traverse underwater sections in Tropical Coast. On the Wii U/PC version though, the player needs the Yellow Drill to enter to enter them. This game marks the first time since Sonic Adventure 2 where the player can traverse waters in a 3D perspective.