A beach is a boundary between solid land and fluid, changing water. A beach in a dream represents a boundary between your pragmatic thoughts and your emotions, between knowledge and instinct.
The tide coming up on the beach symbolizes the encroachment of your emotions and your basic drives onto your attempt to approach a problem in a practical manner.
A beach itself contains neither pure water nor solid ground. It is made up of sand and mud, products that come from the interaction of land and water. Therefore, a beach in dreams represents the integration of practical knowledge and intuition.
People often go to the beach to relax, so a dream that you are on a beach can mean that you need to relax more.