cofferdam

noun3

1. •
   A temporary watertight enclosure used to create a dry foundation for building bridges and other structures over water.

   • The contractor for the cofferdams, &c., was an Englishman, as were the principle workmen, and all the machinery has been supplied from England. — 1847
2. •
   An empty space that acts as a protective barrier between two floors or bulkheads on a ship.
3. •
   (no longer used) Synonym of dental dam (“barrier to keep the teeth dry during dental work”) (dental sense).

   • I refer to Dr. Barnum, who has given us the coffer-dam. I cannot detain you while I describe the immense benefit which his application of a very simple material has conferred on the public. — 1874