From a quick Google search it seems that Mr. Halladay learned to fly in 2013 or 2014.
The FAA’s online Airmen Registry shows HARRY LEROY HALLADAY III holding a Private certificate (the lowest of three levels) with ratings for single-engine land (e.g., Cessna 172), multi-engine land (e.g., Beech Baron or King Air), and Instrument Airplane (“instrument rating,” enabling flight through clouds with assistance from Air Traffic Control). He also had a “sport endorsement” for “Airplane Single Engine Sea,” presumably related to flying the Icon A5. The certificate was issued on November 16, 2016, suggesting that he had been signed off to fly the Icon A5 on that date.
The certificate and ratings that Mr. Halladay held represent significant experience and achievements, so this was not a situation in which a raw novice was at the controls. That said, seaplanes and amphibians tend to require a higher level of skill to manage. For example, if you land an amphib with the gear down in water by mistake, for example, that’s a life-threatening accident. By contrast, a gear-up landing on an asphalt runway is life-threatening only if the flight school owner runs out and strangles you. The airport is a much more structured environment than a lake or bay. You’re not going to hit a submerged log on a paved runway.
The above-referenced USA Today story says that he’d owned the plane for less than a month. One of the biggest factors for safety is “time in type.” A pilot with 30,000 hours may not be safer than one with 3,000 hours total time, but a pilot with 500 hours in a type of airplane will be statistically much safer than a more-experienced pilot with just 10 hours in that airplane. On the third hand, articles indicate that Mr. Halladay had previously been a renter of the Icon A5, so his experience with the plane might have included 50 hours of training and rental over the preceding year.
The Icon A5 includes a flight data recorder, unlike most small planes, and therefore there should be some answers fairly soon about what happened. Seaplanes can be tough to land in glassy water due to the difficulty of perceiving height above the water. Thus it might be interesting to know if the water had been flat calm at the time of the crash.