Squash and cucumbers are good examples. As water deficit continues, the foliage wilts permanently, discolors, and falls prematurely. Lawns will take on a blackish-green cast and the grass blades will not spring back into position as you walk on the sod. In other words, you'll leave footprints. But long before you get to this point, just as the plants begin to show the slightest symptoms, your yield of flowers and fruit has already been compromised.
Excess water can be just as bad.
As the soil becomes waterlogged, oxygen supply to the roots is severely reduced. It's normal to see the wilting and water more heavily. You must maintain adequate but not excessive water in the soil to ensure good growth and survival. But that saying may not be at all accurate or adequate for your conditions. Applied all at once to sandy soil, it will only leach away.