If you're saying that you won't stop swimming then it could be that your space bar is stuck. This happens to me sometimes if I open an application that takes a few seconds to start up (such as skype) and return to playing the game while I wait. I would suggest you unpause the game and press space.
Chat with our AI personalities
sometimes a bug in underground mineshafts but imposible to do by ureself. In minecraft 1.2.5
/toggledownfall to stop or make rain in minecraft
Press the ESC key.
It might be a problem with your mouse. Either that or there is a hardware compatibility issue with minecraft.
Stop holding left mouse button....