Highlight one of your brushes, right-click on it while it's highlighted and make it a func_train. After that, right-click in the main editing window and create 2 path_corner entities. These are basically markers for your func_train to move between. Highlight your func_train then highlight one of the path_corner entities, then hit "ctrl+k" to link them. If you highlight the path_corner and bring up the entity editor, it will tell you it's targetname. With the first path_corner highlighted, go ahead and highlight the second path_corner and hit "ctrl+k" to link path_corner 1 to path_corner 2. From there you need to make path_corner 2 target path_corner 1 so it will be a continuous loop. While in Radiant, with the path_corners set you can highlight the func_train by itself, hit "ctrl+a" and in the camera window, you can see where the func_train will move back and forth while in the game. You will probably need to move the path_corners around a bit to get it set right, but this generally takes no more than 2-3 tries to set it properly. Good luck!
Heheh, I stumbled upon 'floating' items when I made a big bathroom for my project Protosphere....made a huge rubber duck and some bottles 'float' around in a big bathtub. Just link the brush/group of brushes the func_train and set two or more path_corner 's in the water for it. Simple and fun effect. Make sure to target the right path_corner names.