how the large floating leaves have helped the lotus plant to survive in water?

Answer 1

The floating leaves position them above the water, putting them in direct sunlight. This light is essential to drive photosynthesis, which creates physical material for the plant in the end. Having them above the water (or on the water) also gives them access to the atmosphere, especially oxygen and carbon dioxide. These two are necessary for both the photosynthetic reaction as well as photorespiration.

This has allowed the plant to get the nutrients it needs from the air and sun, while having the majority of the plant under water protects it from land-borne herbivores, and the water helps regulate temperature better than air. It works both ways, and is an ingenious way to survive.

Answer 2

The leaves absorb CO2 and sunlight, hence photosynthesis, plus provieds structure for rising flowers.