audible message indicating where the gear is positioned be of help, i.e. the standard phrase "4 blues landing on water, 4 greens landing on land",
These systems are standard on some very modern amphibs, and can be retrofitted to most. When I have flown them, I have not allowed them to distract me from actually visually confirming the gear position, and my intended landing surface, and speaking in aloud twice before landing. My amphibians do nt have these systems, and others I fly do not, so I don't allow myself to change/reduce my vigilance in terms of checking for myself every time. Also, the advanced warning system can fail, and then is silent. "Silent" is a normal condition for other amphibians, so could lead the pilot to not checking for themselves.
No matter what the warning system, or pilot discipline, task saturation can overcome it if the pilot is not vigilant. Landing an amphibian on water is nearly always different than landing at an airport. Nearly always it's uncontrolled, so there is not a controller to assist you (and maybe remind gear, if they know the plane), so you're more attending to other traffic. If it's a busy body of water, "runway" incursions are very likely, and, you're probably customizing your circuit to be non standard to avoid certain features on the ground. If you're landing "away", yes, you probably have the place to yourself, but now for sure, it's not an airport, and you must be aware of everything to do with aerodrome environment - landing area, winds, obstacles, hazards in the water, and where you're going ashore. All of that can demand a lot of pilot attention, and it's easy to forget things. I was training a 7000+ hour airline pilot, and during our umpteenth water landing, he was set up to forget to check. He was surprised when I suddenly called for an overshoot from a perfectly set up landing. I did not explain why the overshoot until we were climbing away - I think he remembered after that!
I teach that the pilot must get everything stabilized at some point before the final approach, and dedicate uninterrupted thinking to configuring the plane correctly for landing. That done, speak it out loud. Speaking it out loud while by yourself is odd, so memorable. Speaking it out loud with passengers, allows them to consider for themselves, and perhaps make their own assessment of the situation.