http://www.turbojet.com.hk/eng/schedule/airport_2.html
First ferry is at 11:30a. You won't make your connection, so have to take the coach.
There is now a coach that goes HKG-SZX via the Shenzhen Bay crossing, so you can avoid Huanggang:
http://www.trans-island.com.hk/eng/china-line_szj.htm
7:45 to 18:45, once an hour, at :45. HK$180.
There are other Trans-Island and CTS buses that go through Guanggang, but the Shenzhen Bay one is clearly way better.
But it's still going to be tight. If you miss the 8:45 bus, you will have to take the 9:45, which should get you to SZX in under 2 hours, which is 11:45. Their website says 3 hours, which is way too much, unless they need you to wait for another bus at Shenzhen Bay. You should get there in 2 hours or slightly less.
[Their Chinese page doesn't give an approx travel time, only the English one. Which is interesting... ]