First scuba gear appears at construction 2. Shamblers Skin (1w) and amulet of breathing (1a) One man gear but enough to send a group of competent commanders into the deep. At construction 4 the first group scuba gear appear. At 6 comes the submariners and at 8 the fabled "Orb of Atlantis" that essentially lets you take on water nations as an equal.
See Zens item reference sheet for information.
http://www.shrapnelgames.com/Illwinter/d2/6.htm