I came up with the idea of making a bathing suit that when it makes contact with the water it changes color.

Consult a patent attorney who can evaluate the patentability of you bathing suit, and can prepare a patent application for you if the results are favorable. However, the broad concept is not new (see, http://www.mermaidinengland.com/home/home-of-the-original-colour-change-swimwear.php ), so if you intend to sell one, you should have a right-to-use evaluation of unexpired patents performed first.