It wasn't one "faulty line", which is understandable. I'm talking 4-5 SA Mastery Series Lines in different sizes and a couple Orvis and Sage lines, which I'm pretty sure 3M makes. If not I stand corrected. All of them have started sinking after a 1-2 weeks use.
Now, most of my clients that fish SA (and there are lots) have few problems. But they're fishing those lines maybe a week a year. However, I had a good buddy down a couple weeks ago and he spent 2 weeks fishing bones, tarpon, and permit here. He always laughed at my problems with SA lines too, until his started sinking... and catching in the grass, coral, rocks, etc. He mostly fished from kayak or beach at home and never had a problem.
I've talked with other guides that do lots of wading, from Hawaii to Mexico, and many have had the same problems. One guide out of Nervous Waters Hawaii comes particularly to mind, but he did mention that there was one year in there that did keep floating... but I'm not sure which.
I want to be fair though. I'm talking about the loose loop of line sinking while you're wading: 4-8 hrs a day for months on end during the season here. I'm not talking about a line sinking after you cast it - the surface tension of stripping the line seems to mostly prevent that. I never carry my lines, nor do many of my clients (that usually results in a tangle when a fish takes off). Those lines have more than enough time to absorb water... but then other brands don't seem to have a problem.
PS I love the way they cast and shoot a mile, which is the devil of it.