This is a guide to making a great white shark costume, out of foam.
Step 1: Make the torso
Get some blue sheet foam, around 10mm thick (roughly, you'll need your chest measurement by about 2 metres, plus half as much again for spare and fins. One can of foam glue (spray stuff), and a strip of white soundproofing foam for the teeth (a metre or so).
Make a tube which goes around your waist up to under your shoulders. this part is going to take some of the weight of the costume, so it should be snug at the waist and sit on your hips. It's simply as a tube of foam, glued together with a 15cm overlap at the front or back.