I am looking to do the exact same thing... Our lake is not that big (~300 acres) and would love to have a 3D printing of it as a conversation piece. I have pretty much mapped the entire lake.
I did connect with Humminbird Customer Service and they mentioned that it is currently not possible to export the 3D map in 3D printer format. Too bad.
Any wizards out there who can create a utility for this???
Thanks!!