This is Westcountry slangdating back to before the First World War. Skates are very slippery fish and the name was applied to people who had slippery natures in that they were untrustworthy, would try to avoid giving information or were just not very likeable. The use of the name expanded to include people who were wheeler dealers or people who do a lot of ducking and diving but no actual work. The name is disparaging but at the same time may hold just a hint of reluctant admiration. It is, however, a warning as a skate has no conscience and is totally unscrupulous and untrustworthy. The term is believed to have been used extensively by the British armed forces and was used to describe someone who was a slippery customer, fishy and who avoided their share of the work.
Nickname given to bales of marijuana thrown overboard or out of airplanes. Origin is reportedly from members of the US Coast Guard in the area of Southern Florida.