Caisson vs Watertight - What's the difference?

As a noun caisson

is (engineering) an enclosure, from which water can be expelled, in order to give access to underwater areas for engineering works etc.

so tightly made that water cannot enter or escape.

(wikipedia caisson)

(en noun)

(engineering) An enclosure, from which water can be expelled, in order to give access to underwater areas for engineering works etc.

* 2003 , (Bill Bryson), A Short History of Nearly Everything , BCA, p. 213:

Caissons were enclosed dry chambers built on river beds to facilitate the construction of bridge piers.

The gate across the entrance to a dry dock.

(nautical) A floating tank that can be submerged, attached to an underwater object and then pumped out to lift the object by buoyancy; a camel.

(military) A two-wheeled, horse-drawn military vehicle used to carry ammunition (and a coffin at funerals).

(military) A large box to hold ammunition.

(military) A chest filled with explosive materials, used like a mine.

(architecture) A coffer.

* caisson disease

* * ----

(en adjective)

So tightly made that water cannot enter or escape.

So devised or planned as to be impossible to defeat, evade or nullify.

a watertight''' contract; a '''watertight regulation.

* watertight alibi * watertightness