And a follow up question: What diameter?

Thanks.

Tim in SC

Any strong non-stretch line will do. The stuff CLC sells specifically for perimeter line is black 1/8" and 1/4" solid nylon braided rope. Over on the rope and cordage page there is also a white version of the 1/8" product.

Ignoring price, there's also all the sail rig lines. Most are ridiculously strong and don't stretch. They're available in different color schemes and cost substantially more than nylon.

Nylon braided rope like the stuff in hardware stores can be a good choice, too. It can come in many colors. Just make sure that it's nylon, not a blend, and solid braid. The price won't be much different from CLC's but the color selection may be better. If it's radically cheaper than CLC's then be very suspicious and check its fiber material. It will probably be a blend with low-strength stretchy fillers.

Finally, small diameters will fit easily through eyelets and stuff, but could be painful to grip. Larger diameters will be more comfortable but could have problems fitting. I've seen some builders slip a larger diameter plastic tube over thin line for use as a handle.

Laszlo

   I use (and sell) black reflective (it has a silver stripe that reflects light in the dark) deck line in either 3/16" or 1/4". The 1/4" is easier on the hands but, as Laszlo says, it depends on the type of eyelet you are using which will work best. I find it swells and shrinks a bit as it gets wet and dries so I usually leave it a little long. This also makes it easier to grab.

Dan (moonlightmarine.net)

echoing everything Laszlo said.

i wanted to add that i have had a lot of luck finding inexpensive cordage that i use for perimeter lines at some of the on-line rope markets such as the examples below:

https://atwoodrope.com/collections/reflective/products/5-6mm-x-50ft-battle-cord-reflective-black

https://sgtknots.com/products/reflective-paracord?variant=33137273045078 

these are two examples of black with relfective elements (one is closer to 1/8 and one is closer to 1/4) that i find particulary attractive for anybody considering kayaking at night or low light situations.  

my experience is that these kinds of sources and these lines (variations of paracord and used in the camping/outdoor community) have held up well, come in all colors and sizes and have shown good price points relative to lines that are sold expressly for marine purposes.  in addition to perimeter line material,  they also have a remarkable collection of bungee cord and strapping material colors too. 

since i consider the rigging, not only a key safety element, but a decorative element as well....i have found that i use these kinds of sources a lot for the variety and price point.  at these price points, i can swap it out easily when i want a new look and give a little fun update to my boats without a lot of money being involved.

h