Don't get me wrong. I love that swimming is a total body workout, but sometimes I miss that intense lower-body burn that I get from a good run or bike ride.
If you want to increase the intensity of your swim workout, especially in your legs, try this. After every three regular laps, do a kicking lap. That means you keep you arms either at your sides or straight in front of you (like Superwoman), and propel your body through the water solely with the movement of your legs. It doesn't matter what kind of kicks you do, but my favorite is scissor kicking so my legs feel like they're working equally.
At first it'll be really tough to do an entire lap of kicking, so start off by swimming as much of the lap with your legs as you can. Pretty soon, you'll find your legs gaining strength and endurance. Not only are these kicking laps great for toning your quads, hamstrings, bum, and lower back, but I've found that when I go back to swimming using my entire body, I feel much faster and and more powerful overall.
Fit's Tip: If your gym has them, you can use a kickboard to help your torso stay afloat. It'll help you concentrate on just kicking your legs.