The trip to the sea represents change in Meursault's life. This change is a deviation to his normal routine, which will put him under the influence of others. The reason that the sea is chosen is that it in itself represents chanage. The sea is never the same; it is constantly moving, and therefore, constantly changing.
Besides just being a deviation from his daily routine, this trip is the catalyst to the end of Meursault's routine forever. It is by going to the beach that Meursault is trapped in the chain of events that will lead him to commit a murder and be sentenced to death. He has little control of the chain of events - they move like the tide, and carry him along with them.
Meursault accepts this. Although others in the courtroom feel he is cold-blooded, he is simply accepting of his lack of free will, and willing to be pushed around by the tide. He has one angry tirade, but in that is able to let go of his need to think like a "free man", and thus he remains passive to the last.