You will want something to anchor the plants. However, stem plants can be left floating eventually forming a mat at mid-level/surface. Rosette plants like aroids, or swords will need something to keep them weighted down if you dont want to put them in some kind of planter.
Now, regarding planters, use a lightweight substrate like clay hydroponics balls, or kitty litter(can be very silt heavy though - rinse thoroughly before use). Another consideration is that eventually plants produce enough root mass to stabalize itself in a container.
A final option is to use suction cups with clips and simply place the plant at crown level (or around the stems in the case of bunched stem plants) in the clip/ring.