This underwater level gets darker as you progress. In order for the darkness to not force you out of the level, you need to capture the various Chinchou and Lanturn in the area. If you manage to get enough points, you'll unlock the Pokémon; Manaphy

Bronze Medal Requirements:
Capture 12 Pokémon and make it to the end

Silver Medal Requirements:
Investigate the area and find the mirage Pokémon; Manaphy

Gold Medal Requirements:
Earn 75,000 points