Over-reaction?

Last night we saw someone swimming in the harbour. He'd probably been out drinking. Two policemen arrived, called to him and shone their torches. They arrested him. The harbour is hardly used. except by a few small boats. The man was hardly a danger to shipping. The water is very shallow. The only danger is of the man drowning, or cutting himself on the rubbish that gets washed up there. I don't see why the man was arrested.