The necessities for any garden are water, nutrients, air, heat, and light. These lead to biological activity which leads to plant growth. In wet or flooded conditions, the excess water crowds out air in the soil, leading to rot and stagnant soil.
In addition, wet soil is constantly being cooled as water evaporates off the surface. This may not be a problem in warm weather. In spring and in the north, it is a major obstacle.
The big challenges are to replace stagnant, anaerobic soil conditions with aerobic, active ones, and possibly to warm the soil cooled by evaporation.