I guess it is your tractor,but building a pontoon and floating out into the pond is not a good idea.Plus how you going to anchor the pontoon when your digging and keeping it from tipping. Where are you going to put the silt? in the pontoon with you?????? Sounds real interesting to me.I'm sure the rental company will like copsized pontoon and backhoe under water.Forget the pontoon idea. If you want to clean your pond use the backhoe to cut a hole in the dam and drain it, let it dry and then try to clean it.Your talking major project here. Just in case you do try this? PLease do send pictures!!!!!!!!! Good luck.