Definitely water weight...your muscles retain water to repair themselves after working out. Your just starting at the gym so the scale is going to be showing some funky numbers the next few weeks or so...Don't worry.
I'm looking at my week weighins right now, and the first week I hit the gym I had a 5 pound gain, followed by a loss of those same 5 punds, followed by regaining those 5 pounds and holding onto them for two weeks...then I started seeing a steady loss...
Be pacient...and a big congrats on starting up at the gym...