Without water, plants wilt and die. But too much water can be as bad for plants as not enough. If land plants are submerged in water for too long, even if just their roots are submerged, they may rot or drown from lack of oxygen.
Balancing plants' water needs is like having a healthful diet. Everything should be consumed in moderation. Provide your plants with enough water for good health, but don't flood them with it.