how do I make a model boat out of metal that will float?

Answer 1

The weight of the water displaced has to be greater than the weight of the object displacing it.

Take a look at river barges. They are relatively simple in construction. Large, flat-bottom craft with relatively thin skin.

See: http://www.route99.org/journal/july/images/coalbarge.jpg
for an example.

(Full page link: http://www.route99.org/journal/july/july27.html with some other information on SW Penna.)