altmtl

If you are taking the public bus then they will take you whatever. If yu are taking the VIP bus then it can get full so booking the day before is advisable. If you need somebody to help you in Ranong then I always use Kompas - 086941 5365. If you call him the night before he will meet you at  the bus station and arrange everything for you including the dollar note and the boat, then take you back to the bus station usually in time for the 10.30. am public bus to BKK arriving BKK around 8 pm. The return  VIP bus only gleaves Ranong at 8 pm in the evening.  All for under 1,000.

I would get the bus if I were you. It wold save messing about in Chumphon.
Be warned - The road from Chumphon to Ranong is no fun for anyone who suffers from motion sickness even slightly.....

I doubt an ordinary bus would do BKK-Ranong in 9 hours, you probably look at the timetable?

Some time ago I did see some directions and advice here on TV.com on how to get from the airport to the southern
bus terminal. If you look/search a bit maybe you find it.

Cheapest would be to take the bus all the way to Ranong.
(A long ride.)

My choice would be to probably spend one night in Hua Hin or in Prachuap
on the way, possibly also a night in Chumphon.
(Umpteen hours on a bus is not very comfortable.)

As for ferries;
Now in late October I expect that they're into high season schedule,
means ferry in the morning and in the afternoon aswell as a number
of speedboats during the day.