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.

Step 2: Shape the Head

make another tube for your head, which will have armholes so it sits on top of your shoulders (this will take some more of the weight). Again, this is just a tube which is glued at the front or back with an overlap, and it should extend about 30cm above your head (you can always trim it down), and overlap the waist section by at least 15cm for glueing them together.

Whilst wearing the waist section and the head section, cut out some armholes and glue the two sections together.

Now you should be wearing a big tube, and you need to shape the head. Basically you need to cut it like a bishops hat (i've posted a pic of one for example) and glue the seam together at the top. I made mine so the front of the head is flat/vertical, and the back came forward to meet the front (you can see what i mean in the diagram). That's the opposite of what a shark looks like, but otherwise i think you'll find your head is poking out of the shark's mouth like it's eaten you feet first (which may also look cool!).

Step 3: Add the Teeth

Add a strip of the acoustic foam all around the inside of the mouth to make the teeth. Glue some fins on the sides and back.

Step 4: Add the Tail, and Cover It With Material (or Paint)

Add a tail; it's just another cone, with some tail fins glued on. Then glue that cone to the waist section. For what it's worth, i'm not sure i'd bother doing a tail if i did it again. it looks enough like a shark without it, and it's a bit of a pain to drag around and sit on.

Next i covered it in blue material, because painting foam is annoying (it uses masses of spray paint because it absorbs it all), and material covers up the seams. But it didn't look great, and if you want it to look better you should use thicker material, perhaps something with some silver sparkle in it.
Add a white chest to the shark, and some black shampoo bottle caps for the eyes.