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As both Sonics can be underwater for good portions of the Acts, air bubbles are also back in the Acts. This issue can be remedied for Classic Sonic if the player has the Aqua Shield equipped.
Gameplay[]
Act 1[]
This section requires an expansion.
In this Act, Classic Sonic takes to the submarine underground in temple sections.
The Kaos from Ocean Palace returns in this Act. There are also underwater portions making allusions to Hydrocity Zone and Labyrinth Zone. A Kao also appears in Act 1, trying to crush Classic Sonic towards the end of the Act.
Act 2[]
This section requires an expansion.
Modern Sonic speeds through this Act along looping alabaster highways, and may choose to either jump along the backs of giant turtles or drive a go-kart on a suspended road.
In 3D sections, running on water is a feature. The moving ruins from the original Seaside Hill return in this Act, as well as the Egg Pawns, with some new enemies like Spinners. Some Quick Step sections similar to ones featured in Jungle Joyride makes an appearance as well. At one point the player can choose to go through a kart section (reminiscent of the Bobsled sections from the original stage) or jump across several turtles similar to Ocean Palace.
Challenge Acts[]
Act 1[]
Rescue the Animals!: Save a set number of captured animals from Capsules and reach the goal in time.
Doppelganger Race!: Face of against Sonic's double and beat him in a race to the goal.
Espio: Way of the Ninja: Press / to have Espio attach a rope to the hooks and reach the goal in time.
High-Speed Hijinks: Enemies move and shoot at double speed. Try to get to the goal in time!
Underwater Challenge: Keep an watchful eye on the underwater terrain and get to the goal in time.
Act 2[]
Rouge: The Temptress: Press / to have Rouge take out enemies using her wiles and reach the goal in time.
Doppelganger Race!: Face of against Sonic's double and beat him in a race to the goal.
Trick Island: Use Rainbow Rings to pull off enough tricks and reach the goal in time.
Surf, Sea, and Sand: Take on various course and find your way to the goal before time runs out.
Terror From the Deep: Skim the water's surface and try not to get chomped by a giant Chopper. Get to the goal in time!
Trivia[]
Seaside Hill Act 2 forbids the player from taking unnecessary shortcuts. The water around the stage is marked off by buoys, and whilst Modern Sonic can run on water, if he ventures outside of the safety area, the player will have approximately 2 seconds to make it back between the buoys or the surface or a Giant Chopper will emerge from the sea and eat Sonic, causing him to lose a life.
Though most likely a glitch, the player can reset the time it takes to eat Sonic whole if the player allows Sonic to drop slightly underwater although it is very risky as the player may be fully submerged underwater and lose a life.
Unlike in Sonic Heroes, Tails and Knuckles are not playable.
As a result, the kart controls are modified. Only Sonic rides the kart, and it is possible to boost while using it.
The kart segment is similar to Sonic & Sega All-Stars Racing, as Seaside Hill was redone into a race course for that game.
This stage can be debated as Ocean Palace as well as Seaside Hill, as it not only contains portions of the Ocean Palace level, but also shares its music in Act 2.
Seaside Hill is the only Dreamcast era level to not have its Modern background music change at predetermined times, as both Speed Highway and City Escape have dedicated sections with new music, while the Ocean Palace theme is integrated into the Modern remix of Seaside Hill.
This level is placed in the Dreamcast era, despite not appearing on said console. This was done to make each era have an equal number of levels.
The Egg Pawn is the only returning enemy in this stage, since the Spinners and Choppers never appeared in this stage.
Strangely, Spinners replace the Flappers as the flying enemy.
In Act 1, if Sonic has the Bubble Shield on, bounce on a certain platform in the water near a big wall and you are able to get to the top. If the player succeeds this and then jump off to the other side to the cannon, the ground in the next section of the level is invisible.
A remix of this level appears in the background of the mobile game, Sonic Dash.