One thing comes to mind in your case may be contacting someone with a Gradall,these machines have a boom that extends to around 40 ft and the one I seen in operation had a 36 inch backhoe bucket as well as 6 ft scraper blade . The pond i seen being scraped this machine (and probably in your case too) couldn't reach the middle of the pond but sure did a great job with what he could reach all while leaving the water in it.
The operator had enough experience that he claimed he could feel the clay liner at the bottom so he didn't disturb that.
Of course I have to think these machines come with a price..lol
Good Luck..Larry