The Center for Disease Control has several suggestions to prevent swimmer's ear.

Raise your hand, how many of you have gotten this? I get it at least once every other year but I suspect the majority of the time it is from ocean related activities.

From the Center of Disease Control (CDC):

Swimmer's ear (also known as otitis externa) is an infection of the outer ear canal that can cause pain and discomfort for swimmers of all ages. In the United States, swimmer’s ear results in an estimated 2.4 million health care visits every year and nearly half a billion dollars in health care costs (1). The good news is that there are a few simple steps swimmers can take to prevent swimmer’s ear.

[...]

DO keep your ears as dry as possible.

• Use a bathing cap, ear plugs, or custom-fitted swim molds when swimming to keep water out of your ears.

DO dry your ears thoroughly after swimming or showering.

• Use a towel to dry your ears well.
• Tilt your head to hold each ear facing down to allow water to escape the ear canal.
• Pull your earlobe in different directions while your ear is faced down to help water drain out.
• If you still have water in your ears, consider using a hair dryer to move air within the ear canal - Be sure the hair dryer is on the lowest heat and speed/fan setting, hold the hair dryer several inches from your ear.

[Link]

They have a list of "Don'ts..." as well. Be informed!