According to the pilot's written statement, he departed a land-based airport and flew to a nearby lake. Being a short and direct flight, he did not use the departure checklist and forgot to retract the wheels on the pontoons. Immediately upon touching down in the water, the airplane nosed over. Post-accident examination revealed the firewall had been buckled by impact. There were no mechanical anomalies with the airplane or its systems that would have precluded normal operations.