Per the title, S.I.U. is the abbreviation for 'Special Investigations Unit'.
Guest star Marco Lopez appeared in 14 episodes of Dragnet (1967), ten of those appearances were in the 4th season. This was his second episode in season four.
Red bellied piranhas are a popular aquarium fish. They live in Brazil,in freshwater habitats. Despite being described as ferocious predatory predators, they are actually scavenger fish, subsisting on river detritus, fish and carrion. They do not normally attack as a school, but rather will swarm as a defensive behavior. Their teeth, however, are every bit as sharp as the media portrays.
The bartender with the eye patch is portrayed by Tom Williams, Jack Webb's right hand man and associate producer of Dragnet and Adam -12.
The policewoman who is brought in for undercover work is asked to dress as a "swinger". The 1960's definition is very different from a modern definition. In the 1960's, a swinger sometimes refers to a lively and fashionable person who attends lots of social events.