Hamok, I to have experienced the Blue & White bus to Hua Hin. We were luckier than you, we actually had a toilet on the bus, but when you went round a corner the door flew oepn (not a pretty site). The day we took the bus was at 10am in the morning and about 38°c outside.
Once on the bus and travelling found that the A/C was about as useful as an ashtray on a motorbike. I had to put up with my GF (whose good idea this was) moaning for the 3hrs that it took to get to HH.
I vowed never to get on one again, and never have. Last time I got the minibus from Victory Monument (bit like F1 drivers), but got there in under 3hrs. Have always got taxis back to BKK at a cost of around 2000-2500 baht. A much better way to do it, the A/C always work and you can always make a stop for the rest room when you want

Always Borrow Money from a Pesimist. "They Never Expect it Back"