Capybaras possess slightly webbed feet, which makes them excellent swimmers. Their natural habitat is in areas with lots of vegetation next to rivers and large bodies of water. They love to swim, and spend much of the day in the water. While they prefer to leave their heads only partially submerged, they can also dive if necessary.
At SeaWorld Orlando, a capybara named Westley is one of the featured Animal Connections in the Sea Garden. These daily encounters allow guests to get up close and personal with exotic animals. It's quite obvious if Westley is coming out for a visit, as they set up her personal pool so that she can enjoy the water. Some days, they even let her show off her swimming skills by setting her free in the nearby pond. This is an amazing opportunity to pet and interact with this surprisingly sweet, gigantic rodent. It's definitely been one of the highlights of my time in Florida so far.
Additional information can be found on Sea World's Animal Bytes entry for Capybaras.