Submerged flower centerpieces are fairly easy to make if you have the right flower hard goods and other supplies. If you plan for the flowers to be completely covered with water, it's best to wait until the wedding day, as you do not want bacteria to begin growing as the flower composition breaks down.
It is better to after securing the flowers into the vase to pour just enough water to allow the cut end to take up water, then adding the rest after delivery.
For this particular look, you'll need the following supplies and fresh flowers:
This arrangement of roses is very simplistic, seemingly suspended in water, these roses stay in place. In actuality, all flowers tend to float, so they need to either be anchored in place with a metal frog pin or suspended in clear gel water beads.
The frog pins are the "traditional method", but can get rather pricey. The water beads are a quick and easy alternative, the the invisible beads actually do well in holding the suspended flowers in place. To achieve the "invisible" appearance of the beads, you simply choose a clear bead and don't pour off the excess water.
These beads take hours to achieve their full size, so you need to add the water to the beads well in advance of the wedding date and seal them in the large containers you can purchase at a local discount or dollar store - even a storage tub will work if don't don't choose too large of one. (The weight can get rather hefty once the water beads are fully absorbed).
Simply place the vase on the table, pour some beads in the vase, arrange the flowers, and carefully add more beads. Don't go completely to the top, as you will need to add water so the beads disappear.
Don't do this in advance of the wedding day, as the water will begin to decompose the flowers. If you elect to use the frog pin, then pin one or two leaves to the frog, and push the flower stems down through the leaves (cutting a tiny slit in the leaf if necessary) and push the stem of the rose onto the frog pins. After all four roses are in place, lift the arrangement carefully and drop down into the vase. Add only enough water to cover the bottom of the flower stems, adding more water after the vases on placed on the wedding reception tables.