The grim reaper is always imaged swing'n a scythe ..... which could be used for underwater, weed removal ....... so's you could use an old field scythe .... to get it done.
You know .... I suspect a scythe could work pretty good in depths up to about five feet, and probably is easier to use in more shallow water depths? A scythe actually works pretty good, as long as it is kept sharp. Sort of like shaving the weeds off the lake floor.
www.digg.com/video/how-to-scythe ..... and, don't forget your flip-flops for staying steady on the uneven lake floor!
Craigslist-nh/farm&garden has a used scythe identical to that in this video for 20-bucks, down in Raymond, NH.