Nothing in this Charter shall be construed to permit the City of Bellingham to sell all or any portion of any of the real property it now owns or may hereafter acquire for water supply purposes in Township 37 N. Range 3 E. of Willamette Meridian, and more generally known as Lake Padden, Ruby Creek and Silver Creek Watersheds, whether or not used for water supply purposes, unless a question of such proposed sale shall have been first submitted to the qualified electors of the City at a general or special election called for that purpose and assented to by two-thirds of said electors voting on such question.