I would look really hard at the sickle bar mower. The flail mower will indirectly still flip grass and weeds into the pond water. The sickle would just lay it down.

I bet you can get an old sickle mower on the cheap at a farm auction.


[Linked Image from corvettejunkie.com]
http://www.pondboss.com/subscribe.asp?c=4


7/8th of an acre, Perch only pond, Ontario, Canada.