Decide on the size of your arrangement. A single large float looks elegant and is attention getting, and a cluster of smaller floats creates a playful feel. Purchase your supplies accordingly.
Hot glue the tray to the center of your Styrofoam block. Cut a chunk of floral foam to fit inside the tray, and glue it in place. Let the glue dry for a day or two to ensure it is completely set before adding any flowers.
Pin banana leaves over the styrofoam base to hide it. If you're using real flowers, fill the tray with water to soak the floral foam. Once the foam is saturated, add your flowers, or simply add fake flowers to the dry foam.
Anchor the centerpiece in the center of the pool using fishing line. Screw in four eye screws on either side of the Styrofoam base and thread a length of fishing line through each screw, tying it securely. Attach the other end of the line to a flower pot or pool skimmer.