As pointed out in a comment by cobaltduck in the question What type of rope to use for the mainsheet of sunfish? the proper name for rope when used on a boat is line.
If I go to the store and buy a length of rope and put it on my boat it is now a line.
When does it become a line instead of a rope?
- When it becomes attached to the boat.
- When I took a sailing class and learned better.
- When I buy it and it does not belong to the store any longer.
- Or ...