Done this trip last year on a vip bus from Hua Hin they stop a couple of times at air conditioned restaurants and you have a set meal included in the price of your ticket they also give you snack boxes and refreshments on board the coach and there was continuous DVD's.
As i remember we caught the coach by the clock tower around 7.30 8.30 arrived at the ferry port around 16.30 had to wait about an hour and a half.
The ferry crossing was a really chilled out experiance, we stayed the whole journey on the upper deck by the funnels, but no two days are the same at sea.
On our arrival at Samui it was around 20.00 hrs be aware that the next ferry carrys Haz cargo i.e gasolene and other general freight so there is no tourists on the next night crossing.
My friends drove me around the whole island there are some nice things to see like a waterfall with elephant rides, a mummy fide monk in a glass cabnet very strange but true, and he has a pair of black sun glasses on.
My recommendation would be the Ark bar on a tuesday or a wednesday they have a beach party very cool place to be.
A word of caution tho the roads on Samui are busy but really poor many are concreate and very bumpy i see loads of accidents with tourist falling off mopeds etc.
Kendo.

Is Bangkok a place or a nasty injury.......Eric Morcombe.
Proud to be a Southampton FC Fan.