The Myth - A sinking ship creates enough suction to pull a person under if that person is too close (as was rumoured to occur when the RMS Titanic sank).
Verdict - Busted
Notes - Though using a small ship, neither Adam nor Jamie were sucked under when it sank, not even when they were riding directly on top of it. The use of a vessel with a large displacement was not practical.
It was noted during the episode that the story of Charles Joughin, the Titanic's chief baker, contradicted the myth. He testified in a 1912 enquiry that he held onto the stern railing of the ship as it went down. As the ship went under, he stepped off; his hair did not get wet, much less himself get sucked under with the ship.
Later, while preparing for the Ping Pong Salvage myth, the Sinking Titanic myth was re-tested and re-busted as Adam pointed out.
I read that bubbles rising from a sinking ship can create an area of water which is less dense and people would sink in it.
The Mythbusters could easily test this by checking buoyancy in a stream of compressed air in a water tank.