The cheapest is skin on frame style boats (like the original Inuit kayaks.) You build a frame with wood and then wrap it with canvas and paint it to keep the wa

The cheapest is skin on frame style boats (like the original Inuit kayaks.) You build a frame with wood and then wrap it with canvas and paint it to keep the water out. People have even made small dinghies this way. The easiest (maybe, SoF is pretty easy) are probably plywood boats. A lot of people like "stitch and glue" but the "glue" is usually epoxy resin which is a human carcinogen so I've never tried it.