Wll, the knot is invariably pronounced bo-lin (in England). In the song "haul away the bowline" the rope is pronounced bo-lin.
But in canal circles, the actual rope is always(in my experience) pronounced bow-line as Dave Ruch suggests. people would look at you oddly if you yelled "Can you take my bo-lin". It's bow-line, and stern-line (or starn-line).. But this is now, in England. Then, on the Erie Canal???? Pass!