Chuck, it's good to see you posting again.
I particularly liked Bill Cody's idea of siphoning down the water level, getting a long reach track hoe in there to dig out the slurry, pumping with a trash pump more than once, etc.
It sounds like a project for July or August though. I'll come up and help if you like.
I have some diving experience too, just not cold water, dry suit stuff. If you could solve the questions of getting a muck pump, I'd like to try the underwater diving style of removal.
What we prolly would have to do is to get our Pond Boss Diverman Snrub to leave his farm in Kansas and visit Hotchkiss to assist. LOL
Do you have places on your property to spread the muck? It could be a really good soil amendment if it is primarily organic matter.
Roger