American Airlines doesn't charge any extra for waterskis - http://www.aa.com/i18n/travelInformation/baggage/baggageAllowance.jsp - That means if you're a Gold or above Advantage flier they're free (no charge for other bags either). If you have to pay for bags, you'll need to pay the "normal" bag charge for the skis. Rules are as long as the bag isn't more than 125" total (length + width + height which means that most jump skis won't be free unless you find a counter agent without a tape measure!).
Now it does specifically say a "pair of skis" - Last September I went to the counter and said it was a "slalom ski" and she asked if it was a pair. I said "no, just one" and they wanted to "check the rules".....I said, "oh yeah, I actually have a pair of slalom skis in there". Problem solved.