"Cowichan Lake (not Cowichin nor Cowitchin)" adopted in the 7th Report of the Geographic Board of Canada, 30 June 1908.

Source: Canadian Geographical Names Database, Ottawa

The name "Cowichan" is that of the confederation of approximately 11 loosely-affiliated Salish tribes that occupied the Fraser River delta, the Gulf Islands, and much of southern Vancouver Island. Their name means "the warm land," and has special reference to the Cowichan Valley. The Cowichan's own name for the lake is "Kaatza" meaning simply "the lake". See also Cowichan Bay and Cowichan River for origin information.

Source: BC place name cards, or correspondence to/from BC's Chief Geographer or BC Geographical Names Office

The bay was named about 1850 by officers of the Hudson's Bay Company for the powerful Coast Salish First Nation - the most populous in BC - that occupies this territory (and now spells its name Quw'utsun'). Cowichan Lake, Cowichan Station, the Cowichan Valley Regional District, etc, all derive their names from the same name source. Cowichan is an adaptation of an Island Halkomelem term for "warming the back," which, in turn, refers to a bare, rocky formation on the side of Mount Tzouhalem, said to look like a frog sunning itself. There were many variants of this name in the old days, includeing Cowichin, Cowitchin, Cowitchan, K'au'itchin and Ka-way-chin. The earliest version recorded probably that of the Hudson's Bay Company trader John Work, who mentioned the Coweechin in 1824. In fact, so many Quw'utsun' people migrated across the Straights of Georgia each year to occupy summer village sites on the Fraser and harvest its great salmon runs that Work identified that river as the Coweechin. The Songhees First Nation name for Cowichan Head, the northern boundary of their territory, was Tiumalatchung.

Source: Scott, Andrew; "The Encyclopedia of Raincoast Place Names"; Harbour Publishing, Madeira Park, 2009, pp. 137-138.