Interesting topic. I suppose the best thing to purchase would be a crystal ball. Then you'd use your motor as intended whilst following all the manufacturers recommendations. Then, after how ever many years, just before it completely clogs up, overheats and dies, you'd use one of these descaler products and not be overly concerned about a little bit of pitting or etching of your aluminium casings as you've hopefully had many years from it.
Just for interest, this marine engineer I was chatting to today, told me of a small Yamaha outboard he'd had from new in 1996. It had been used for one season and flushed after every use.
After that, it was stored indoors, serviced every couple of years and ran up 2 or 3 times a year in a bucket of tap water.
When it was approx 20 years old, he sold it to a pal.
First time out it overheated. When they stripped it, the cooling system and associated parts were completely corroded.
Like I said earlier, lack of use is the worse thing for any motor!