The private pilot was ferrying friends to a lakeside lodge in his float-equipped airplane, under Title 14, CFR Part 91. He said he landed too close to the beach, and was unable to stop before the airplane impacted the beach. According to the pilot, the airplane traveled up the beach 20-30 feet, and the left float structure collapsed. He said the left wing and left horizontal stabilizer struck the ground, damaging both. The pilot said there were no known mechanical problem with the airplane prior to the accident.