I had a Cortland 333 6wt line on my lake rig when it went overboard, and sunk
to the bottom. It worked fine for me, and I had no complaints. If you didn't
have your mind set on the Rio Mainstream, I'd say the 333 should work just as
well. If you want to make a point, give the seller negative feedback (assuming
he doesn't offer to send a new line, and cover shipping the 333 back).