If your boat is properly wired, why would you need reverse polarity protection on the DC side at all???
The first time was when I had taken all of the batteries out and started putting them back in and momentarily wired the first battery in backwards. That little mistake required sending the Icom 706 off to the manufacturer to have a diode (I assume) replaced.
The second time was when I purchased a piece of equipment from Europe that was positive earth and I didn't know that, it got wired up wrong, and that sucked current backwards through the 60 watt power supply on a computer. That one was harder to replace because they didn't make it anymore, so I had to modify the way the computer was working to work with a new power supply.
The third was simple slip of a screwdriver while working on something that smoked a capacitor in one of my own boards, that one was easy to fix because I had built the board myself.
The fourth was an automobile amplifier that ... you get the idea ...
I would not want to suffer the current drain of any of these reverse polarity solutions all the time, and so it is not my intention to have them installed all the time. What I am doing is wiring up a new fuse panel and I want to be able to switch the reverse polarity protection in and out on the more expensive equipment whenever I am doing battery maintenance, making modifications to the electrical system, etc, to limit damage that I myself cause. Once the system is stable then I can go back and check everything with a volt meter and take out the protection at the fuse box as part of my final procedures for maintenance.
I doubt that someone who paid other people to do electrical work would need reverse polarity protection, but someone like me who is constantly working with the system makes mistakes because the system changes all the time. I am always making modifications, and accidents just happen. Accidents have happened with me quite a few times now and each time it ends up costing me time and money.
I may end up leaving reverse polarity protection in place on a few things. The Icom radio, for example, is expensive, and it uses so much current when it is on that a little extra current wasted charging a relay coil isn't that big of a deal. Since I really have harmed that radio before, and it is a very important piece of equipment, I might protect it all the time.
That was the long answer.
The short answer is .. "If your boat is properly wired .." is a BIG IF.
Edit - another way to say that is that for most people "electrical system" is a noun, but for me it is more of a verb ...