Longer water fins is an excellent way to train lower body strength and cardiovascular endurance. You may also use cardio sets and add upper body or core exercises. See our excellent lower body sculpting water fitness workout called Finning: the Ultimate Butt Burner Workout (DVD021) for an awesome challenge!
There are short training fins available should you need less intensity or drag resistance in the water.
Various sizes and colours are available. Pick the option closest to your shoe size.