3-strand nylon for
dock lines, unless you're on inland waters - you need the stretch, but don't go too large diameteror you lose it again.
Watch out for wet laid nylon springs
heating up and losing tensile strength. Eqally don't go so oversize that your teeth jar abunk, as each spring stops stretching.
For fenders I use sections of old worn
dock line - and I never use my
safety rails, always back to cabin-top handholds. I've seen too many capsized supports.