Disagree that a single amp powering everything at its absolute limit can tax it's power supply and heat sinks of that amp. A quality amp it doesn't matter, it's been designed to be used in a non ideal environment (tucked away somewhere on a boat) and can take the abuse and can produce it's rated power for more then likely longer then the batteries on the boat can last.
So here are a few things to consider.
When you half the impedance load, the amplifier produces more power but it does not come close to doubling the power, even though Ohm's Law states that it should. That's mostly a product of the amplifier's power supply and it's fixed limitations. The DC to AC to DC switching power supply contained within all amplifiers, and not the boat's supply. So batteries aren't part of this equation.
When you test one channel driven in isolation, and then test all channels driven, all channels driven will produce less power per channel. This is solely a product of the amplifiers internal power supply reaching its limit.
When you run at the lowest permissible output impedances the amplifier runs less efficiently. It consumes more power at the supply end as a ratio of what it delivers at the output end. The difference is just wasted heat that the heat sink must dissipate.
The lower operating efficiency and greater ratio of heat to power taxes everything.
Can you get by running the amplifier at its permissible limit with all small coaxials run in the highpass mode? Sure. Those smallish speakers with tiny voice coils don't represent a monumental challenge. But when those channels are comprised of more difficult and more complex workloads such as subwoofers and tower speakers that are typically run at the threshold of clipping, the amplifier challenge is very different.
Do some amplifiers have a self-protective rollback feature? Sure, the better ones do. Which keeps the user from destroying his equipment or exceeding the heatsink dissipation capacity.
This is an open-field environment where the equipment is driven significantly harder than in a car/truck/SUV. And there's no air conditioned compartment in a towboat.
Heat kills electronics. Running electronics at their thermal threshold for the long term is damaging. Maybe the equipment still operates but the audio quality is eventually compromised.
People in the industry understand these issues and look at it realistically, and not they way they hope it is.