"We are going to meet at the beach."
Translation:E launa ana kākou ma ke kai.
Most common in conversational Hawaiian as well as in traditional writings is "i kahakai" without an article. However, "i ke kahakai" does exist, but it is usually used when the name of the beach (or its location) is also included, such as "i ke kahakai ʻo Waikīkī," when just "i kahakai" wouldn't work.
Same goes for "ma kahakai" and "ma ke kahakai," although use of "ma" is not as common as "i," and it cannot be used if it means "to."
Hope this helps.