Part of the confusion expressed arises because there are many types
of marine lines (most "ropes" become "lines" on a boat, but that's
a peculiarity of another expensive pastime). :-)
There are two major rope construction methods: three-stranded (long wound
spirals) and braided (a smoother, woven exterior, similar to kernmantle
Typically, synthetic fibers are used in marine lines. Some, like nylon,
are stretchy (in a relative sense) under heavy load. Others, like Dacron,
Spectra, and Kevlar, stretch less. Polypropylene floats and doesn't
pick up much moisture. (In its common three-stranded form, polypropylene
line is inexpensive and somewhat hard on the hands. A more expensive
braided form is soft to the touch. Neither absorbs much water when
wet and both dry quickly.) The different fibers vary in sensitivity
to UV exposure and abrasion. Still other factors influence design
of climbing, caving and rescue ropes.
For thicknesses likely to be comfortable, strength and durability
will probably be moot points in selection. Flexibility, softness
to the touch, friction on sliding, uniformity of feel when wet
and dry, and thickness would all contribute to your satisfaction.
(Some of the marine lines are available in a variety of colors
and or patterns).
IMO, no hardware store is likely to be very knowledgable regarding
alternatives among the available marine lines and climbing, caving,
and river rescue ropes. Nor will they carry splicing tools that allow
you to make compact eye-ends in the braided ropes to attach snaps or
REI and various outfitters carry climbing, caving, and river rescue
ropes. Boating suppliers (including BoatUS http://www.boatus.com and
West Marine http://www.westmarine.com) carry marine lines.