Ocean Palace, as the name says, is a palace in the middle of the ocean. It is similar to Seaside Hill and has the same platforms, robots and everything except the palace-like ruins.
Story
After making it to the Whale Island, the heroes discover an ancient sea palace. Team Sonic and Team Dark chose to flee from the palace to pursue Eggman and smash his robots, destroy collapsing pillars, ride on giant, rock-carrying turtles and eventually end up having to outrun three Kaos (giant, circular, boulders with demonic-looking faces and spikes sticking out on the side) and make it to the Goal Ring at the central palace. Team Rose is being dragged along with Amy by her hopeless obsession to find Sonic and must find the Goal Ring in the ruins floating on the sea. Team Chaotix must rescue a Chao from the palace on the orders of their mysterious client. The Egg Hawk is fought on a road and several small islands in the ruins near sea level.
Rank requirements
The following is a list of requirements to attain a rank for each mission with each team.[1]
Normal Missions
Rank A
Rank B
Rank C
Rank D
Rank E
Mission 1
Team Sonic
50,000
45,000
40,000
32,000
<32,000
Escape from the ancient ruins!
Team Dark
90,000
85,000
70,000
55,000
<55,000
Flee from the ancient ruins!
Team Rose
40,000
35,000
30,000
25,000
<25,000
Find the ruins floating on the sea!
Team Chaotix
50,000
45,000
37,000
32,000
<32,000
Rescue the captured Chao!
Extra Missions
Rank A
Rank B
Rank C
Rank D
Rank E
Mission 2
Team Sonic
45,000
40,000
35,000
30,000
<30,000
Get to the Goal Ring within 5 minutes.
Team Dark
5:00
5:30
7:00
9:00
>9:00
Defeat 100 Enemies (Time needed minutes.)
Team Rose
5:30
6:00
7:30
9:00
>9:00
Collect 200 rings (Time needed minutes.)
Team Chaotix
43,000
40,000
30,000
25,000
<25,000
Rescue the captured Chao without being detected by the enemies.
In Mario & Sonic at the London 2012 Olympic Games, part of Ocean Palace is used for Dream Fencing. It uses flags with the same designs as the stone pillars, and the bleachers are located in the sea.
Sonic Dash
In Sonic Dash, Ocean Palace (named Temple Zone) appears as part of the never-ending stage.
Trivia
Elements of this stage appear in Seaside Hill for Sonic Generations. After the player listens to the background music for Seaside Hill in the Modern level, the music then switches to Ocean Palace's music.
For some reason, the player can never see the Kaos falling into the water after they outrun them in the chase part of the level. The player can only hear them falling into the water pit between the chase area and the central palace entrance in which the Goal Ring is located. If the player tries spinning the camera while in the air to see them falling in, it would result in the player either falling into the water pit below them or getting hit by the Kaos in mid-air, causing them to fall into the water pit.
This is another one of the levels from the various games in which Sonic (or any other playable character) gets chased by something.
The far background image used for the stage is an aerial photograph of Lighthouse Reef located off the coast of Belize.
Interestingly enough, if players use glitches to get to Team Sonic's area with Team Chaotix, the Goal Ring still has the gold star in its center, instead of a back arrow.
Ancient civilisation established on the sea... 
