Late response I know, but I was searching the forum for Thailand and came across your message. My experience is that as long as you come across as competant in the pre-sail briefing, they won't have any problem with qualifications etc. Just be constatntly aware of fringing coral reefs sitting off beaches - they rise up from the bottom very rapidly.
This time of year you really want to be in the Gulf of Thailand rather than on the West coast. AFAIK
the only charter fleets on this coast are Sunsail out of Koh Samui and Ko Chang. Both are nice, but don't offer the solitude they offered a few years back.
I will be chartering on the other side (in the Tarutao group) in October - this is even too early on that side, but we have decided to take the risk. We will charter from Langkawi in Malaysia, and head to the islands from there