Actually,
Fenders are meant to protect the boat
and dock from damage while tied up.
Bumpers are those little rubber thingys that are intended to be dangled over the side of the boat, swinging merrily to and fro while the boat is underway, and serves to give the boat a more nautical look. How you look is very important in todays world.
BTW:
Underway.. is another nautical term meaning.. the boat is no longer tied up to the dock and this is when the FUN starts... OR.. it starts to get really Scarey.
NB