I dived under the Top Hat yesterday, we just bough it this week. I noted that the marine growth on it was totally different to what grows in the Georges River. It is easy to do a comparison as both vessels were antifouled within a month of each other abour 14 months ago. The Top Hat (near Manly) has no slime, it did have tube worm looking things, a orange lichen looking thing and some barnacles. The Botany Bay vessel has thick slime (usually does) and barnacles.
To me it appears that the water clarity of the Manly mooring equates to no slime. The BB moored vessel gets thick slime probably from the murky water. The keel can sometimes touch the bottom on a very low tide, yet I have in 10 years never seen the bottom of the bay there.
One other factor that may influence the growth, is the antifoul. I use Altex #5 ablative, generously applied (min 3 coats, more in strategic spots). The Top Hat appears to have a hard antifoul as when I scraped it nothing came off into the water, unike the ablative versions.
If the just the water quality is the reason for the difference, then paying a bit more to be in the seaward bays of Sydney harbour is probably worth it with less frequent antifoul jobs, no slime and clear water to dive under the boat for a regular clean. Oh, the chap I bought the Top Hat from said he had not cleaned the hull since the last antifoul job.