The most amphibious races seem to be aventi (Stormwrack, p34), shoal halfling (Stormwrack, p45), river spirit folk (Unapproachable East, p14), and sea spirit folk (Oriental Adventures, p14). Those will all require some modification if they're to be used in a Pathfinder game. Well, maybe not shoal halfling, since it's defined in terms of regular halfling.