I hope this isn't thread necromancy but as it happens, I had to learn all this recently. I figured that is the OP isn't still around, this is still useful information and might be helpful to somebody else.
Water will tighten the canvas but it's very temporary and will slacken again in short order. Sometimes it may seem like it stayed tight but in those cases, it's due to humidity changes, not because the water-spray trick works long-term. Keys work a little better but they stretch in a way that is bad for the paint and primer and will eventually cause radiating cracks from the corners. It may take years but it will happen. These fixes are half-measures which are not permanent, unfortunately. Not to mention, humidity, heat, etc. will cause some variation in tightness of a picture no matter what, unless you are keeping them in a climate-controlled museum-grade archive! That's why it's so important to keep the corners of stretcher bars a little loose and never glue or nail them together and is why you have to avoid stapling the canvas corners in a way that crosses the join.
The good news is, it's really not that hard to fix "properly" if you have a screwdriver or something similar and some pliers to pull out staples (without damaging your canvas of course). All you have to do is pull the staples near the sag, pull it tight enough to make it taut and neat, then put in a few staples again. It's really not hard and thick primer or paint won't be harmed unless you really
overdo it. You don't need to be strong, I have seen very small women do it and I am not exactly a weightlifter myself
The hard part is simply getting over the fear of pulling apart a painting and messing with it, it takes a little boldness and I was scared off from that for a long time. All for nothing, I was pretty sheepish when I did it, thinking,"that's it?!?"
Just remember, all the framing and stretcher bars and staples and tackle are, at the end of the day, just temporary and completely malleable to whatever you need or want. The stretcher bars are not part of the painting and you can easily take the painting off and restretch it at any time you feel a need. Long term, if you expect your work to be around many years from now, it will have to be restretched in any case!