Best way to simulate an outside underwater area?

[Diago21]

Hey guys,
I'm making a map that takes place inside, however the outside can be seen from various points throughout the map and I'm just wondering how to make the outside area look like the one from the main game. Eg. The tunnel area you walk through in Upsilon at the start of the game. Advice on settings, skyboxes, etc. would be much appreciated!