I don't think you believe them that the 6x6 will be too hard to drive. Let me use numbers to help you understand ... If you look at the area of the pipe you are driving approximately 1.62 square inches of area compared to 36 sq in... That is 22.2 times more difficult to drive in.. If you drive the pipe with a post driver you apply around 100 lbs per hit, to drive the 6x6 post you will have to apply 2220 lbs per hit to drive it... I doubt you're that strong, so the 6x6 will sit on the mud and provide no support...

I really like the idea of building on the ice, I wish i would have done that... Instead I tried to do it from a boat and my posts ended up crooked. I just drover mine with a post driver rather than washing them and it worked okay except for not being vertical... I think if you combine the two ideas you can build a great structure and it will last a long time... I haven't had much luck with treated lumber lasting when submerged lately, unless you get the creosote treatment in ties... Another reason I'd go with the steel well casings at least they last 40 or more years... Good luck... Later J