Usually people take the 23.00 or 01.00 buses from the clock tower. They'll give you enough time to be back in Hua Hin in the evening. Never done it this way myself, though, so I hope somebody else will confirm this.
This info may be too late for you - however.
There's a bus that leaves the main terminal in town at 7.40am. Get there early as there's no way to book ahead. I've never known it full though. Arrives Ranong around 6 hrs later - 2pm ish.
The motorcycle taxis are now attuned to what's happening and will take you to immigration, then to long tail boat, wait for you, back to immigration and then bus station for about 150/200 Baht.
As long as you don't stay too long shopping in Burma, you're back in time for Thai immigration. Can catch, say a 4pm bus and be back in HH around 11pm.
By the way, you have to go to the mainland now after the island, armed with your passport and receipt for 5 dollars. They don't stamp you at the island any more. Guess the Burmese want you spending a bit more.