If you have no reverse polarity protection and you want to add it, you can usually do that effecitvely for under $5 in parts. You install a zener diode just before the point where you want protection (i.e. on the DEVICE side of the plugs) and then install a fast-blow fuse just upstream of that, closer to the battery, in the positive line. (Although either line would do.)
You choose a zener that has a reverse-voltage rating just high enough so it doesn't conduct in normal use. Then if the cable that has the zener installed in it is connected backwards, the diode conducts, acts like a short circuit, and blows the fuse.
This is called a "crowbar" protection circuit, because it literally acts like someone has thrown a crowbar across the power lines and blown the fuse. the hardest parts are finding just one zener diode at a reasonable cost, and installing it correctly. You get it backwards, you blow the fuse immediately, you reverse the diode and now you know it works.(G)
OR, it your device can tolerate a little less than full power supply, you can install a simple full-wave rectifier (bridge rectifier) in the device side of the cable. That way you are pretending the cable is supplying AC, and ensuring that only the "correct" DC will get to the device, no matter how it is plugged in. That's another $2-3 part, the onloy drawback is that it will reduce your DC supply voltage by something like 1/2-1 volt.
The crowbar appraoch doesn't reduce your supply voltage at all.
If any of that sounds like Greek to you, any marine radio or radar tech should be able to come install it for you in 1/2 hour or so, any tv repairman if you can still find one of those.
Thanks for pointing this out, I actually design computer hardware and have done some 12V design. So now that you started the discussion indulge me on a long post. It's funny because I actually hadn't really thought to go down this road because I wasn't happy to be dealing with it. A good connector is all I need but it's really tricky to find the right ones.
Note with the perko connector you can't actually seat the connector in reverse, but nothing stops you from touching the contacts and briefly shorting in reverse. Also there are no visual indicators so trial and error will result in a reverse 50% of the time.
As for the diode crowbar you don't need the zener unless you want to protect against over-voltage as well, a regular diode or schottky diode (rated for >12V which almost any diode would be) will do the job (positioned with the arrow pointing at 12V so with normal polarity it doesn't conduct, with reverse polarity it does). Anyway, again the connector makes it so easy to connect up-side down I can't blow a fuse every time this happens.
The rectifier is a good idea that would solve the problem but as you say, it cuts the voltage by two diode drops. The advantage of the rectifier would be that the circuit will work correctly no matter what the polarity which would be great if the connector did in-fact seat in reverse. However since it doesn't it's not quite required.
If I were completely unwilling to tolerate voltage drop one solution would be PFET transistors. Unfortunately the body-diode would conduct in reverse so you'd need two back to back. Mosfet's can also be a bit fragile however there are great integrated MOSFET power switches which protect themselves against everything - temperature and over-current. 12V Power would pass drain to source to source to drain with both gates tied to what is supposed to be GND. If it's connected correctly both PFET's would turn on and there would be near 0 voltage drop. In reverse they would be totally off. Small solid state relays and/or other integrated solutions probably do exist that would make this even easier.
Note that whether a voltage drop over the diode matters depends on the load. Few devices will run off 12V directly so they need to convert the voltage down. If they do it with linear supplies they just burn up excess power. Sending them a lower voltage actually has 0 impact (until a point). However if they use more efficient switching supplies, the load current will depend on input voltage and will go up if voltage goes down. In this case the diode will matter. Given that the autohelm is mechanical I might guess it's running it's motors off 12V directly in which case a reduced input voltage will have a negative impact.
....But I think my best bet would simply be to put a schottky diode in series with 12V on the Autohelm. A good schottky will only drop ~0.3V.
For electronic stuff Digi-Key is a fantastic source (basically the McMaster of electronics). However, of course none of it is designed for marine applications specifically. Digi-key actually has plenty of connectors rated for harsh environments but wading through their selection, the selction of each end, the pins, the strain relief etc can be a huge pain which is why I went with the perko.
Here is a diode that's a candidate (I filtered for through hole, 20V-60V, 10A to 30A), however let's audit it's supposed 12A rating. At 12A it specifies a 0.55V drop which would be 6.6W. This is a lot. It specifies that it rises 10.5 degrees C for every watt so that means it will rise 70C or 160 degrees F!, at 80 degrees ambient this is above boiling, not good. However the part is rated for 150C so it can technically handle it. I'll check my autohelm to see what it actually draws, it may be a lot less than 12A average. (Note, adding another diode in parallel isn't very effective)
Digi-Key - SB1245CT-ND (Manufacturer - SB1245)
An example of an integrated power switch:
The perko connector in question: