There are many useful plants that simply float on the surface of the garden pond, which are really useful and decorative. There are also a few that can be classed as weeds - if they are not controlled.
The most well known of these being common duckweed! But even the invasive duckweed can be a great help in cutting down the algae-forming ultra violet rays from the sun. Together with creating cover, they are a myriad of short roots below the surface, which are a boon for newly hatched fish. They find refuge in the root system of the floating weed!
There are two basic kinds of floaters - large and small leaved floaters! The large leaved floating pond plants are typified by the water chestnut or water hyacinth. The small-leaved floating plants are typified by the duckweed as above. Many of the small leaved floating plants are invasive, and will have to be kept under control on a regular basis.
Invasive is also a good description of the Water Hyacinth - Eichornia crassipes seen in the image. However, if you keep it under control, it is a superb floating plant for your garden pond. Any offsets can be donated to other pond lovers - or even exchanged for plants which are not so invasive!
Bear in mind that floaters - invasive or not - can do much to help your pond stay healthy. In particular, they can prevent a lot of sunlight - which is essential for algae growth - from reaching down into the pond. Together with that, they are good hiding places for small and newly-hatched fish fry.