The term 'slender swimmer' can be broken down into two crossword solutions: EEL (3 letters) and GAR (3 letters). Eels are a type of fish that are long and slender in shape, and they are found in both fresh and saltwater habitats. GAR are also a type of fish, but they are more closely related to the bowfin and the paddlefish. They have long, slender bodies and are usually found in freshwater habitats. Both eels and GAR are excellent swimmers, making them ideal for the term 'slender swimmer'.