The airline transport certificated pilot reported he was landing on a beach at the conclusion of a Title 14, CFR Part 135 on-demand flight with six passengers. During the landing roll, the nose wheel encountered soft sand, and was deflected sideways, resulting in structural damage to the firewall bulkhead. During a telephone conversation with the NTSB investigator-in-charge a day after the accident, the pilot stated that there were no preaccident mechanical problems with the airplane, and that the landing was uneventful until the nose wheel sunk into the sand.