Why are the leaves of completely submerged aquatic plants divided or dissected

Your friends are correct!

The submerged leaves are often highly dissected or divided. It is an adaptation of leaves.

• It helps to create a very large surface area for absorption and photosynthesis. 
• It also minimizes water resistance and hence potential damage to the leaves

@ Others: Good answer!! We really appreciate your contribution to this forum. Keep posting!  Cheers !!