Our '85 boat came with a rub rail one of the previous owners had installed that is working out really nicely. It's a dense plastic that is a wood substitute that park services use to build modern picnic tables etc. out of. It's like a 1x4 plank, very dense, flexible, and I think you could hit it with an ax without doing much damage.
I'm very familiar with Starboard. I've just replaced the clothes rod holders in all my hanging lockers with Starboard pieces I cut. In my opinion, the plastic wood stuff is way better for outside rub rail use. It looks okay, too.
I don't know what specific brand is on our boat, but I suspect there are several options. Doing a quick search for "plastic lumber" will bring up listings like this one:
and this looks a lot like what we've got. It's really tough and totally maintenance
free. So there's one option for you.