The pilot of the amphibious airplane reported that, during a no-flap water landing, he noticed a higher descent rate than expected. He added that he applied full power to initiate a go-around but that the airplane landed hard on the water. The pilot and passenger egressed the airplane and were rescued without further incident. The airplane sustained substantial damage to the fuselage. The pilot reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation. The pilot reported that he believed the airplane encountered "a windshift/shear to a tailwind as [he] transitioned high to low for landing approach toward the [south-southwest]." A review of recorded data from the automated weather observation station located about 6 miles west of the accident site reported that, about 17 minutes before the accident, the wind was from 090° at 9 knots.