OK DaddyO
How about 25 years if removing "beard" stain every time it re-appears? Several times every summer, is that long enough? I have tried a number of different brands of toilet bowl cleaner, all of which contain hydrochloric acid in some concentration. The ones that work best simply contain more HCl. My "beard" doesn't re-appear any faster now than it did when I first got my present boat in 1994, so I don't believe there is any damage being done by the cleaner.
About 20 yrs ago a friend of mine gave me a can of one of those expensive "marine" preparations (that don't tell you how much HCl they contain) and it didn't work as well as the better toilet bowl cleaners.
That same friend didn't need the cleaner any more, as he had just had his boat Awlgripped. His boat still doesn't get a "beard", proving that the only really effective remedy is Awlgrip. If could afford it, that is what I would use.