Visa arrangements are fairly simple in Thailand just so long as you do not step outside the limits. Likewise with the protocol of dealing with the Thais. Do, however, bear in mind that this is a sharply distinct culture and read the advice below for tips on how to best get by during your stay.
Visas & Permits
On arrival, if passengers don't have tourist visas they will be granted entry for one-month. Extensions of seven or 10 days can be made. Crew get a non-extendable one-month entry — if you previously obtained a visa from Thai diplomatic missions abroad, it will be ignored — and are technically in-transit with the vessel, meaning they will not receive a white departure card in their passport and must each post a 20,000-baht bond if they wish to leave the country without the boat.
For this reason, it is simplest if only the skipper actually signs in as crew, while others enter as passengers.
Permit extensions - Foreign-registered yachts are permitted to remain in Thailand for up to one year. Yachts are given six months on arrival, which can generally be extended for an additional six months. For most, a letter, in Thai, from a marina proving paid moorage is sufficient. For those not staying in a marina, a good reason is required, such as needing critical boat work. Some have used letters from contractors listing work that needs be done.
Customs will require photos of the boat and work required (an engine in pieces). Some have reported that they were able to obtain an extension without a letter of any kind. The situation could change at any time.
Note: the above content provided courtesy of 8north.com
Dress - When dealing with government officials or doing business in town, it's appropriate to wear clean and slightly more formal clothes - no T-shirts or flip-flops at check-in time. Untidy or unclean clothes are considered an insult. For men, a collared T-shirt or shirt, shoes or sandals, and long pants are preferable.
For women, it's a good idea to have the shoulders and thighs covered. A blouse with skirt or trousers is ideal, and is comfortable for buses, etc. Just look around at the local people; they're nearly always very neat and clean. Follow suit and you should have no problems.
Behaviour - It's not wise to get angry or impatient, to swear or shout. It's you who loses face, and it's likely to make the situation even worse. Be polite and attentive, have patience with any language difficulties, and you'll be surprised at how smoothly things can progress.
Remember, government officials have a job to do and responsibilities to carry out, so it's sensible to treat them with appropriate respect. If you're really having problems, try and get to speak to a superior. Remember, you're a guest in Thailand.
Attitude - The Thais are friendly, helpful and generous of spirit. They believe in enjoying everything they do, whether it's work or play. So if you make your business or social encounters enjoyable experiences, you also benefit. As in most Asian countries, it's best that no one loses face in any encounter. Don't argue either yourself or the other into a corner. It will only produce a stalemate.
As when bargaining over price, they start high, you start low and you meet in the middle. Everyone goes away happy. Also be respectful of the numerous religious and royal icons that you encounter. They are held in reverence by the local people.