After some confusion about whether the Gulf of Thailand ferry will be cancelled or not, the marine department in Pattaya has just announced that the service will be officially launched on 16th January 2017. According to Thai Rath, the ferry will take up to 339 passengers between Pattaya and Hua Hin. The maximum speed of the catamaran ferry is 32-34 knots and is expected to make the 113km crossing in about one hour and forty minutes. The road distance is 345km. The first trip leaves Hua Hin at 8:30am. The return journey leaves Pattaya at 3:30pm. The price of the ticket is 1,250 Baht per trip. It is estimated that 70% of the passengers will be people from tour groups and 30% independent travellers.
UPDATE 01: The ferry service will be available free of charge from 1-15 January 2016. After that, passengers will be charged 1,250 Baht. A second ferry, with a capacity of taking 260 passengers, will be added in March 2017. (Source NNB)
UPDATE 02: I changed the official start date to 16 January 2016
I will update this page once I get more information. Looks like there will only be two trips per day.