As has been mentioned before, build it heavy, or you will be deparately trying to shoehorn lead into the hull to get it somewhere near its waterline. I built my Springer out of stuff I had to hand, so it has 6 mm ply sides, 10 or 12 mm bow and stern pieces, and a balsa planked bottom, because that's what I had. (The balsa strips are laid across the hull, so no problem with following the curves, and I put a layer of glass and epoxy over it to stiffen it up and protect the hull when I put it down on hard surfaces.)
One thing I found was to ballast the hull so that it sits bows up when stationary. Then when you get going it levels out but doesn't try and become a submarine.
Good luck, and enjoy your build.
Greg