Question

how excessive supply of water can harm the crop

Answer 1

some plants need only little water for their growth.When you over-water those plants, the water will stagnate and the plant does not get its air supply.The more it stagnates, the weaker it becomes.
It also makes the plant rotten.

Answer 2

Some crops need a lot of water to grow but some need only little water to grow.For example rice requires more water but sunflower don't.Just like that overflow of water may harm the crop.So crops like sunflower should be grown in flood free areas to be harm free.And excessive supply of water may harm the plant.