The show is based on Dragon’s Den, which began in Japan in 2001. It’s likely that they took the concept of going into the dragon’s den, and came up with shark tank.
Of course, it can also be a play on words. In business, sharks are those who are dishonest or dangerous in some way. This can be seen on the show to a degree.
After all, business is business. The sharks will sometimes make a low offer, hoping to get a greater return on their investment.