If I recall several have used mechanical means to skim FA and DW using nets , weighted ropes etc as skimmers. I don't recall a motorized skimmer. Skimming will work to remove but it will keep coming back. From the last link above :

Watermeal can be removed by ranking or seining it from the pond's surface but it is so small it is very difficult to physically removed.

Watermeal tend to grow in dense colonies in quiet water, undisturbed by wave action. Often watermeal will be associated with colonies of duckweeds. Watermeal can be an aggressive invader of ponds and are often found mixed in with duckweeds or mosquito fern. If colonies cover the surface of the water, then oxygen depletions and fish kills can occur.



From http://www.naturia.per.sg/buloh/plants/duckweed.htm

In temperate climates, Duckweed survives the winter by producing buds (called turion) that sink to the bottom of the pond.

Duckweed have the highest growth rate of any higher plant. In ideal conditions (high levels of nitrates and phosphates), the surface area covered by duckweed can double in less than 2 days. The Indian species, Wolffia microscopica, can bud off a new daughter every 30-36 hours. Thus one tiny plant could, theoretically, in four months produce offspring equal to the volume of the earth!

AQUATIC PLANT MANAGEMENT http://aquanic.org/publicat/state/il-in/ces/lembi.pdf describes mechanical control and lists companies with products.