A seed is essentially an embryo, like a baby growing in its mother's womb. However, seeds are created dormant, with the cells of the embryo nearly deactivated, with very slow and minimal biochemical activity. When the seed is placed in conditions suitable for life(such as warm temperatures and in water), it's biology speeds up. It absorbs the water, and its biological processes start-- it starts metabolizing oxygen in cellular respiration, it starts dividing into more and more cells, and eventually it bursts forth as a growing plant. Just like the human body, most of a plant is composed of water(the interior of cells), and thus having a ready source of water is essential for the plant to begin growing and expanding.