An otter's body is built for swimming. Its tail is long and powerful, used like a rudder. Its fur is so thick (about a million hairs per square inch) that water can't reach the skin - the otter stays warm even in icy rivers.
Otters mostly eat fish, but they will also eat crabs, eels, frogs and even sea urchins. They are wonderful divers - they can stay underwater for around 30 seconds, and even longer if they are chasing something tasty.
Otter cubs spend their first three months learning from their mum. She has to teach them to swim, which is funny because they aren't naturally good at it at first. She carries them piggyback through the water and shows them how to dive.
If you ever want to see a wild otter, the trick is to be quiet and watch the water at dawn or dusk. They poke their heads up like little periscopes. They are shy - but in Ireland's clean rivers and rocky shorelines, they thrive.