Which One Should Be Used?
'On the beach' and 'at the beach' are both correct and can be used to refer to activities that are done near the water. However, there is a subtle difference between the two: 'On the beach' is used to describe the physical location on the sandy area while 'at the beach' is used to describe the broader location of the beach area.
On the Beach
'On the beach' suggests that the person or object is physically located on the sandy area that is adjacent to the water. It emphasizes the contact or proximity with the beach itself. Have a look:
She was reading a book
Here, the subject is doing an activity that is done specifically on the seashore.
There were a lot of people
At the Beach
'At the beach' suggests that the person or object is located in the general vicinity or area of the beach, but not necessarily on the sandy area itself. It emphasizes the broader location of the beach, including the surrounding environment. For example:
The subject can be anywhere around the beach.
There is a great seafood restaurant
In the Beach
'In the beach' is incorrect and cannot be used because the beach is an open area and there is no enclosed space within it. Use 'on the beach or 'at the beach' instead.
He plays volleyball
He spends most of his time
He watches the birds
in the beach.