World 3-A (New Super Mario Bros.)
From the Super Mario Wiki
World 3-A is the first secret level of World 3 in New Super Mario Bros. The level is a coastal level, featuring Skeeters. This level is unlocked by paying five Star Coins at a Star Coin Sign at the beginning of World 3, allowing the player to play this level in favor of World 3-1.
This level starts with Mario jumping onto several floating barrels, while avoiding Skeeters. After passing the Red Ring, there are several moving platforms. Mario needs to jump on the last one while avoiding the Skeeter to get onto the next platform. After that, Mario should hit the ! Switch to activate the red blocks for him to continue. To get the first Star Coin, Mario needs to either let the Skeeter use a bomb to destroy the blocks that block the Star Coin, or he can go into the water, and swim low enough so that the player can go under the blocks, but not low enough to lose a life. To get the second Star Coin nearby, Mario needs to jump off the Koopa Troopa below the coin. After going through the Warp Pipe, Mario can get the third Star Coin by going into the next Warp Pipe and, as Mini Mario, he should run on the water and jump to the Star Coin. After this, Mario should go past the moving platforms to the final Warp Pipe, reaching the flagpole.