Removing creases from an oil painting canvas
Stretching it properly on the right sized frame should be enough to remove the creases, no water required. Properly is:

Procuring a support of the right dimensions. The original frame on which the work was painted would be the best.
Centering the painting on the support, and tacking (stapling) the work
to the frame at the midpoint of each edge. This will be on the back of the work so put something on the table to protect the painting. Or, more challenging, do not place flat on table at all instead keeping in on the edge of the frame and only resting on the unworked edge of the painting under it on that side.
Stretch and tack (staple) the painting outward from the centers balancing the tension by working one center to edge, then the opposite side center to edge, and so on...

It may require some force to get the right tension but do not use too much or you will twist the frame. When stretching raw canvas water does help loosen the fibers and will dry much tighter and smoother. However it would be best to avoid water of any type on the finished piece. It could cause damage possibly staining the work, separating the paint from the canvas, encouraging mold and so on.