The Floating Gardens requires the Undine soul as found in Castlevania: Aria of Sorrow/Inner Quarters.
You can find the save point at the bottom-right section. Aside from this, there are two places of interest.
Clock tower[edit]
When you first enter this region, you will want to reach the clock tower. From the bottom-left corner, jump on the platforms and continue up. Take the top-left exit, and keep exiting the following rooms on the left hand side. After passing through a few rooms, you will eventually enter a bridge to the Clock Tower
When you are in the sub-rooms, you can also find $500 in the upper path for most of the rooms.
The black door[edit]
The other location of interest is available when you take the top-right exit within the main section, and continue moving right. You can also reach it by taking the top-left exit in the main section, proceeding left for two large rooms, and turining around at the third.
From here, you can jump down and defeat the Kicker Skeletion to gain a kick attack during a double jump. You can proceed down but the progress is blocked by a black door. You will need to complete Top Floor before attempting to get past this.