Getting the underwater volume in units of 1000cc will give you the total weight of the boat in Kg.

A fairly accurate way is to work out the cross-sectional area at suitable distances along the length, then work out the section volumes and add 'em all together. A scale drawing on graph paper and count the squares can get you near enough the area of the sections, or break the section down into rectangles and triangles, work out these areas and add them together. For the volume of the sections, add the area at each end, divide by 2 to get the average area, multiply by the length of that section and there you are. Then add the volumes together.

For a modest size, Bryan's empirical method is best, but it does need a hull.

I can't remember the name of the series (Salvager or something like that) but there was a guy on TV who made scrap furniture out of railway sleepers, coach bolts and bits of battleship. He did a later series terrorising the French, when he made a "boat" from the fibreglass roof of a Transit van. This could be either warning or inspiration.