Apparatus for working under water

A sub-sea capsule comprising a hollow body which is adapted in its position of use to surround a sub-sea installation and be at all times water filled with the pressure of water in said capsule varying between pressure equal to external sea pressure and substantially atmospheric pressure depending on whether the interior of said sub-sea capsule is exposed to external sea pressure or to substantially atmospheric pressure of a transit capsule connected to said sub-sea capsule, said sub-sea capsule being resistant to external sea pressure when the pressure of water filling said capsule is at substantially atmospheric, means associated with said capsule permitting mounting thereof in sealed relationship around an installation, means carried by said capsule for cooperation with complementary means on a transit capsule to form access means between said sub-sea capsule and said transit capsule, and leak detection means located within the upper part of said capsule for detecting any leakage floating up from an installation through water filling said capsule when the latter is in its position of use surrounding said installation.