First, you have to stop and think, what are my realistic alternatives here. The weeds are there because there are X amount of nutrients in the pond just waitin' to be taken up by some sort of green growing thing. Those nutrients can be taken up by submerged pondweeds. They can be taken up by floating weeds such as waterlily or duckweed. Or they can be taken up by green algae in the water (the ones that turn it into pea soup). Unfortunately, rooted plants around the edge of the pond, such as cattails, do not take up very much of their nutrients from the water, so they're not real relevant here.
Thus, given the amount of nutrients you happen to have in your water, your choices are basically: the way things are now, or covered in duckweed, or stinkin' pea-green algal soup.
Personally I would recommend sticking with what you've got now

I fyou want a clear area to swim in, use a rake (for deeper areas, you can rig things to pull w/chains from shore, or rake onto a boat although that is tres tedious) and just keep at it whenever it starts to grow back in again.
Anything you can do to reduce nutrient inputs to the pond will help limit weed growth. That means, do not fertilize around it, or anywhere the runoff can get into the pond. And make sure it stays well aerated in the summer -- I don't know how deep your pond is, but if it is not real deep and gets pretty hot in the summer, the lower layer of water can run out of oxygen which causes phosphorus to re-dissolve out of the sediments, whereupon it promotes plant or algae growth. A little bubbler pump or windmill type thingamajig can help keep the bottom from stratifying out and running out of oxygen.
If despite the above you are really certain you want to get rid of the pond weeds, see if grass carp are legal in your state, and if so get an appropriate number (probably just a couple for your size pond). They will eat the weeds. You will be left with a muddy bottomed pond that strongly resembles pea soup and has a tendency to become stenchy in summer and possibly cause swimmers itch. Enjoy it if that's what you want ;P
A blanket of duckweed will also kill off the pondweeds, at least to a large degree. But you SURE won't want to swim in THAT, and it is incredibly hard to get rid of duckweed. Even if you have ducks <g>
Good luck,
Pat, aquatic biologist in former life
