If soft potatoes mean it started to dehydrate, then would they plump up again if I put them in water?

Some, slightly-wilted produce items can indeed benefit from a process called ‘crisping’ – items such as leafy greens and some root vegetables can find new life when submerged in clean, lukewarm water. The cells open up and through osmosis, will absorb a certain amount of water (considering that most produce is 90+% water). Following a good 30- minute or so soaking, these items are allowed to drain in a rack, then promptly held uncovered in refrigeration overnight. The following morning the items, much like cut flowers in water, will be crisp and re-hydrated!

Potatoes? Not quite the same thing. Fresh tubers (potatoes) are different, having about a 79% water content, and their physiology (being a living organism, even in storage) does not allow for this osmosis to occur. Shriveled potatoes can still be safely consumed, however, provided there remains some firmness.