You know the swayey things that are water doodads? There you go, there made to gently float like they were in water, used specifically in water. So, if you made a crate float, youc ould put it in water and it would gently sway on the currents, looking awesome.
If you want them to float in the air, you could give them the float ability, go to the main window, then hold ctrl + Page Up and Down and move them up and down. That's what I did with my orbs, and it looks rather awesome.