Water in my back tire
Do you mean water is in the tire - between tire and tube - or in the tube?  The only way for water to get in the tube would be via the valve (or, very rarely, if you rode the punctured tire flat for an extended period).

Water between tire and tube is no big deal -- should work its way out quickly.  

Inside the tube should probably be drained (requiring removing tube from tire) and would be most likely due to water in your pump (or a compressor used to fill the tire).  Cycle the pump a few strokes before putting it on the valve, if you've ridden in the rain recently, and make sure any compressor you use is drained regularly.