At first I thought it is "at the beach" then after a quick Google search I found out that it is flat out "on the beach". However, I have found one instance where a woman, presumably a native speaker, uses "at the beach". I have not done a thorough search to check for more.
What I want to know is whether you consider "at the beach" wrong, or acceptable but uncommon? I am asking you because the Google ngram viewer shows a slightly growing trend of using "at the beach" since around 1947.
When talking about the beach, there are times when one could use either "on" or "at" without much change in meaning, but other times where there would one would be more preferred than the other.
Ngrams won't be very useful when trying to figure out the "right" preposition, because there are plenty of contexts where one preposition might seem more natural than the other – even though the two might be almost interchangeable in other contexts.
For example:
They built a new cottage on the beach.
They built sand castles at the beach.
Generally speaking, on the beach is roughly equivalent to at the seashore. The first sentence doesn't mean that the house was atop the beach, it means the house was near the seashore – that it had an oceanfront view.
The second sentence is describing the geographic location of the activity, more so than describing the location of the property. Using on wouldn't be incorrect, but I'd strongly prefer at in that sentence.
In the third sentence, either word could be used. I've spent the last several minutes trying to formulate an explanation, but was unsuccessful: no matter how I dissected it, I was able to come up with counterexamples that would refute my explanation.
Bottom line: Even when confining the analysis to "the beach," sometimes the two prepositions are more interchangeable than others.
Both of them mean the same, but with subtle difference.
On the beach is a general expression.
At the beach puts a little more emphasis on the fact that someone or something is "at" the beach.
For example, when people go to a beach and have some typical activities on (or at) the beach, a typical expression is on the beach, e.g. We spent time together on the beach. The use of at will emphasize the place, as in Where were you an hour ago? I was at the beach.
Both can be correct; "At the beach" means in the general vicinity of the beach. "On the beach" means on the sand itself.
