Swimming The Styx

Least Invocation; 2nd
By channeling the fiendish and infamous river, you become an aquatic creature. Your hands and feet become slightly webbed, granting you a swim speed equal to your base land speed (and all other benefits derived from a swim speed, including a +8 racial bonus on Swim checks; see MM 311). You gain the ability to breathe water as well as air. This invocation lasts for 24 hours.

Source: Complete Mage