When a cartoon dog plunges into the water, that is called scuby diving.
It is a pun, or a play on words, regarding the word scuba diving. Scooby is a famous cartoon dog (Scooby Doo) so instead of saying scuba diving, which is a type of swimming, the pun is to use the word scuby which resembles Scooby's name to refer to the dog.
