I genuinely don't understand why you find electric eels in particular so problematical. Electric eels' ability to generate electricity is a truly remarkable evolutionary adaptation -- a real-life superpower, effectively. It's unsurprising that a company like the film's Oscorp, which is heavily involved in transgenic research, would be interested in the potential of electric-eel genetics. I would speculate, not having seen the film yet, that the "vat" might be filled not merely with water, but with some kind of mutagenic medium, or perhaps is undergoing some kind of process designed to extract the eels' electrocytes in some way, or whatever.
Of course it's fantasy, but there are many superhero and supervillain origins far more implausible or simplistic -- including Electro's origin in the comics. How can you get lazier than "struck by lightning" as a source of superpowers? At least the Flash's origin involved a bunch of chemicals. Max Dillon was just working on power lines. Why are you not condemning that for its laziness?