Marine archaeologists have discovered a 17th-century shipwreck in recent months with a cargo that includes the world's earliest pocket calculator -- a wooden carpenter's rule -- while exploring the seabed of the English Channel. It offers a tantalizing taster of treasures that may lie within nearly 270 wrecks that have been identified, but whose survival is under serious threat from 21st-century trawlers working the busy channel between the Continent and Britain.

Some historic vessels that fell victim to the sea or cannon...