I'll make this easy and cheap!
First, SubMariner is right, cycling is a great way to tone up your legs. Now, to work on your lower legs, get a pair of old runners or shoes. Make a hole in the toe, large enough to run some cord through it. Tie a 5 Lb. weight to the toe, and let it hang just below the shoe. Sit in a tall stool, on the edge as far as you can go, with your upper legs parallel to the floor knees bent. With the shoes on, do full extention toe raises, rotating at the ancles. This will isolate the muscles in your shins, and combined with the cycleing, will make your swimming easier, causing you to use less air and better enjoy the dive.
Their, cheap, easy, and only takes a few minutes a day. (except for the cycling, should be at least 1 hour of moderate exertion.)
Hope this helps. Give it at least one month of daily exersize, don't overexert yourself, and you will see measurable results.