In some cases, you don’t even need dry land! One of the most effective core exercises I’ve ever done involves taking the medicine ball with you into the pool (preferably not one of those old school leather med balls, though). Push off the wall on your back while holding the ball above your upper chest with both hands, and dolphin kick to the other end of the pool. Try to feel how your core initiates and powers the dolphin kicking motion, all the way through to your feet.
I typically do a set of 50′s, alternating 50 med-ball dolphining / 50 fast fly or back, working the SDK’s. I use a 4-6 pound medicine ball, but you can make it easier or harder by using a lighter/heavier ball or by holding the ball closer/further from your chest.