Here are a few videos I have come across in the past of setups like you are talking about.
Here is a pretty neat barge mounted setup that would be really cool on your pond. He has some other videos of it on his channel as well.
http://www.youtube.com/watch?v=wm1RNr3IYPA
Here are some more ideas.
http://www.youtube.com/watch?v=F75Duztdnrg
http://www.youtube.com/watch?v=xUVfiRyjGdE
There is a video of a really well built overhead rail gantry crane setup that I saw in the past on a youtube from a big trucking meet somewhere in Europe. I cant find it now, but it was a nice custom built job that was loading material in and out of big storage bins. If you can find that I think it would make a great setup for off loading barges. You could build piers up out of the pond and mount a rail high up on columns then just build your carriage to run on those rails using train parts.