That "coming back for it" line is such a classic. I read somewhere that most of the junk on beaches never gets claimed, people just say that when they get caught. You did the right thing hauling it out, tide doesn't care about his excuses.

How often are you actually measuring though? I grabbed one of those cheap checkers too and the trick is checking it every few hundred miles, not just when the shifting starts feeling off. By the time you feel it, the damage is already happening to the cassette. My buddy rides maybe 80 miles a week and he checks his every other Sunday, takes ten seconds. Chains are cheap, cassettes and chainrings are not, so I just swap mine the second it hits 0.5 and call it done. Nine thousand miles on one cassette says the old guy knew what he was talking about.