This is sad, but not new, nor the last time this will happen. For those not familiar with Amphibians, recall that the only possible landing gear warning system is a speed based voice advisory of x gear position for landing on y surface, ultimately, the pilot must get it right. If the warning is ignored, or the system fails, it's back to the pilot's self discipline. In the case of a system failure, there will be silence - silence is a horrible advisory! For this reason, I'm not a fan of the advisory systems. I would rather that there be nothing, and a pilot who will state the landing gear position and surface out loud to him/herself while visually verifying the gear position, and looking to the surface. What it means is that the pilot must take a ten second step back from focusing on the flying, to confirm configuration. Unfortunately, water flying can introduce many variables not found in airport flying, which can draw the pilot's attention further away from the vital configuration confirmation. Though one might fly a conventional circuit around the intended landing surface, and be able to position prelanding checks at the appropriate places along the circuit, a landing situation might disallow a conventional circuit (topography), and that fact will further distract a pilot away from a normal routine. Ultimately, a pilot must remain just scared enough of getting it wrong, that that fear overcomes complacency. I have landed three different amphibians (two of them mine) in the last month, and each time I do it, I get more fearful I'll have a brain fart, so I get more vigilant. It just takes once though. Generally, one could simply retract the gear after every takeoff, to assure that the default position is retracted. However, this too will allow a pilot to ignore the pre landing position self advisory. It's just better to be in the habit for every landing.
When one watches the numerous videos of amphibious floatplanes landing wheels down in the water, and going over, it's interesting to note that it's not the mainwheels contacting the water which sends the plane over, but rather the nosewheels. There was a set of photos of a fellow landing a PA-18 amphib in which one mainwheel was extended, and the other three wheels retracted. The photos show that he made it work, but it looked like a heck of a ride! My flying boat mentor tells me that a few Lake Amphibians have remained upright after a wheels down in the water, but it was still a shocking stop, and ruined plane.
And, speaking from very unhappy experience (though not associated with a wheels down in the water, and I was not flying) being violently hurled into the water when your expectation was a smooth slide out is absolutely shocking. Water at landing speed is very hard, and after hitting it all wrong, either you have to find your own way out immersed and inverted, you were ejected, or you drowned. The only things you will have with you in the water will be those you were wearing at the instant of leaving (either voluntary or ejection) so wear your lifejacket for every overwater flight, and carry a whistle, light, and put your cell phone in a waterproof bag, either in your pocket, or clipped to your lifejacket. It's no good to you later wet in your pocket. The waterproof bag for mine worked, though it was torn from my lifejacket when I went through the windshield. My wife phoned it, and it rang for two days in the bag at the bottom of the lake, then succumbed to the deep forever.
My most confident amphibian students were those who came to the training already self disciplined to state the landing gear position of RG wheelplanes they flew. The water is a new, exciting and distracting environment, so best some habits already be ingrained in the pilot's actions. When you overlay another system failure, traffic (air or marine), or excited passengers, it's easy to forget.
I feel for this pilot, as the others to whom this has happened. They did it to themselves, but it's so easy to forget this vital step, I will never point a finger at them - it could still happen to me. We can all make a mistake, just wheels down in water becomes an instant, irrevocable, total mistake, with little mitigation, other than getting out alive, and adding you voice to the need for good pilot self discipline.....