I hate to be the bearer of bad news, but the growing feet isn't due to water gain (although that be an additional discomfort)... growing feet are due to spreading caused by a combination of the hormone relaxin in your system and the added weight on your feet... and, so, your feet will likely REMAIN larger after your pregnancy. Okay... and if I didn't scare you enough (LOL)... it may even be that hard core exercisers are MORE likely to go up a size or so (I've known a couple of instructors who haven't had kids yet who've said that their feet have gotten bigger) because of all of the pounding, etc.
Susan
(whose feet started at a size 8 before baby #1... grew to a 9 by the end of the first pregnancy, and are now a size 9 1/2 at 27 wks. pg in my 2nd pregnancy IN SPITE of the fact that everyone told me that they only spread out the first time... YIKES!)