Underwater Basket Weaving is a method of creating willow baskets, by soaking them in water.
The term is usually used for college or university courses, that are appearing as useless and absurd, like social studies.
These courses are often associated with athletes and sportspeople, who got into higher education, by the merit of their athletic capabilities, not their intellectual prowess.
Underwater Basket Weaving appeared on the pallet of US universities and colleges in the mid-20th century, especially the 50’s.
Students soon started to refer to apparently useless and superficial courses as Underwater Basket Weaving, due to the sheer absurdity, of what one associates it with; namely scuba divers, creating woven baskets, while in the depths of the sea, or campus pools.
In the second half of the 1900’s, the phrase started to spread further, as a reference to superficial courses, as well as utilizing it for activities, most people see no sense in, yet are practiced by several others.
Urban Dictionary has several articles written on the subject, the first one being submitted to the online dictionary in 2005.