The beach is divided in few sections, The Juhu choupati, a mini choupati, and the rest of the beach.
If you need some quite time, you can head towards the Novetel hotel, and enter the beach from there. Its mostly a bit dark there, but safe. you can spend time there till midnight, and can see few people around.
The main choupati is too crowded, I mostly avoid it, there are lost of sellers , people sitting on the beach, kids playing and the lots of lights.
If you want to take a dip in the water, you surely can, but don’t expect it to be clean, you might find plastic items, garland, flowers, clothes etc floating in the water. So if you wish to enter water, chose the choupati part, as it is a bit cleaner.
You will find many fast food eat outs at the choupati, and carts selling corn-on-cob, popcorn, etc.