It would first be flattened like a soda pop can, and then sink like a stone.

There is no way that a large ship could be constructed to be effectively hollow, and withstand normal air pressure if you pumped out its inner air. Not even nearly. Work it out (something like 10 tonnes/sq metre, right?)

But suppose you could build a super-strong ship and empty it of air. Suppose it were 100X20X10 metres in capacity, 20000 cubic metres. Right?

At roughly 1.2 kg/m, that would amount to about 24 tonnes, a sizeable lorry load, or something like 5 African bull elephants or a lot of big Mars bars.

In fact hundreds of years ago people worked this out and someone suggested that we could have ships floating in the air, lifted by hollow metal balls with the air pumped out of them.

I leave you to work out why it never could have worked.

And what we do instead nowadays.

Have fun,

Jon

i think ships are a lot stronger than soda cans 

the perfect vacume pressure is -14.5 psi ( 1 BAR), yes over 10 ton / meter^2 

but the water pressure on the outside of the hull is 10ton / meter^2 at a depth of 9m ( 1 Bar ) in salt water , and ships dont use thinner plates out of the water line , so would only 2x the pressure differnce that the ship was designed to take  , normal engineering puts  in a factor 5x when humans are expected to use , so it might dent a few panels and suck the deck down a lot  but the structure should hold 

so now if "air " weighs 1.29kg/m^3 ( at 101.35kpa and at 0c temp ) every m^3 of air removed  will remove 1.29kg in weight  so the ship will sit higher in the water ( not by much ) , the hull is still the same shape and displacing the same amount of water

when humans exhale our rib cage's get smaller displacing less water causing us to sink 

OK, let's pull in my horns a little.

I over-stated the "no-way" one cold build a ship to stand evacuation. Georg is correct to point out that submarines are examples of ships that are designed to similar stresses, only much greater. But your average surface vessel is another matter, particularly the parts above the water line. Try emptying a large tin can, such as might have contained treacle or motor oil (if anyone still supplies oil in tin cans!) seal it and pass in a needle through which you evacuate it with a reasonably good pump, or possibly even use tricks such as displacing the air by boiling suitable liquids inside, then seal it and cool it. You will be surprised at how much damage even a 1-bar pressure can do.

If you wish to model a super-tanker in similar tinplate, you need to make it about a metre or two long if you wish to match its strength to the scale of the model.

And I am sure you would not wish to ruin your nice model by evacuating it!

There is a "Stratospheric Test Chamber" at the Brooklands Car and Aviation Museum in Weybridge, Surrey, UK. It is there because Vickers Aircraft (subsequently British Aerospace) had an aircraft factory on the site from 1915 to about 1989. Vickers made the Wellington, Viscount, Vanguard, and VC10 (amongst many others) on this site.

The Chamber was built on the authority of Barnes Wallis (bouncing bomb, Wellington, etc) in the 1950s to test the engineering constraints of new aircraft designs, and was used for many years. It could simulate flight at 70,000 feet (-60 deg C and about 80% reduction in air pressure). It is not big enough for a complete airliner airframe, but it would take a whole Sea Vixen or Schimitar, or say the cockpit and front half of a Concorde fuselage. There was also a supersonic wind tunnel on site.

http://www.brooklandsmuseum.com/index.php?/explore/the-stratosphere-chamber/

Because of the strength required to resist a vacuum in such a large object, Vickers Aircraft commissioned Vickers Submarines at Barrow to build the Test Chamber. (I swear the next part is true.) Vickers did not want the disruption of the build in situ, so they assembled the chamber on a nearby hill, with a slipway. When it was complete, they held a ceremony, complete with a bottle of Champagne, and launched the thing neatly into place through a hole cut in the side of the building.

An illustrative video from the past.

 Why is it important to properly vent a tank car before emptying it?

http://www.last-word.com/content_handling/show_tree/tree_id/2771.html