Definitely Season 4, Spike is chained in the tub and somehow he or Buffy breaks a pipe and the bathroom floods. They escape out the window but can't get back in because the door is locked & Giles is out of town. That's all I remember.

Thanks in advance!

kittyfajitas
Think it's Wet and Wild by Chelle http://dark-solace.org/elysian/viewstory.php?sid=132

Thanks, Chrissel! I don't know how I missed that when I was searching at EF.