How do I swim?

How do I swim? Hey guys i just posted my first let's play Terraria video on my Youtube channel and i was wondering if I'm doing something wrong. I can't seem to swim, any time I'm in water I have to sink to the bottom before I can get back up (It happens at some point in the video) is it a glitch or am I doing something wrong? https://www.youtube.com/watch?v=lIm36U9FAX4