Use cloth baby diapers.
Not a viable option in any California saltwater environment. Certainly not on a 2 or 3 month cleaning frequency.
I thought he might be talking about fresh hard bottom paint that needs time to cure.
Anti fouling paint does not need to "cure" after the boat has been splashed. This is a wives tale. Once the bottom has been painted, it's good to go. The reason it is not advisable to clean a newly painted bottom for 90 days is simply that the paint does not need to be cleaned. Copper-based anti fouling paints have a proscribed rate at which their biocide leaches out. This release rate spikes very high when the paint is new (negating the need for cleaning) but drops to a level after about 3 months that it more or less remains at for the duration of its useful lifespan. This is going to change soon however, as California is going to further regulate not only the amount of copper that anti fouling paints can contain but also the rate at which it is released.
We have Petit Trinidad antifouling paint - works well, but even in cool Bay Area waters the bottom needs cleaning every 2-3 months (knotmeter more often - ). I was paying a diver, but want to do the job myself.
The question I have is - what is the best way to clean marine growth off the hull - without scrubbing off too much of the anti-fouling paint?
The answer to your question is: Always use the least abrasive tool to clean your bottom that will get the job done. What tool this ultimately turns out to be is largely dependant on the age and condition of your anti fouling paint. You have Trinidad on the bottom now and that is good, as it is the best anti fouling product available for use in California, IMHO. If it's new, clean your bottom with a white 3m Doodlebug pad. If it's not so new, you will have to move to a more abrasive scrubber, like the blue Doodlebug. If it's pretty bad, a brown pad will do the trick. But whatever the age and condition, more frequent, gentle cleanings will extend the paint's life and reduce the amount of copper you are introducing into the water column than less frequent, more abrasive cleanings. Brushes, scrapers and other implements are not used by experienced hull cleaners who know how to properly maintain anti fouling paint, except to remove hard growth. In the Bay Area, a 2-month cleaning cycle is considered optimal.
If you are serious about sailing on a clean bottom, you will forget about using a "Dri-Diver" or other version of a scrubber on a stick. Not only will you not be able to clean a large portion of the hull (forget about the keel) but those areas you can reach will likely be poorly cleaned at best. Further, you will have little control over the pressure applied to the scrubber. This means if the paint needs a very gentle touch you won't be able to provide it or if the paint needs some real elbow grease to get it clean, that ain't happening either. 'Course, how will you know if either condition exists, since you are working blind from the dock? Thru-hulls, transducers, running gear and zincs? Not getting inspected or replaced and certainly not getting cleaned.