Quick answer. Thailand is not in the EU, so there is no free movement here. To work you need two things: a Non-Immigrant B visa to enter for work, and a work permit from the Ministry of Labour. A tourist entry does not allow work.
Do skippers and crew need a work permit in Thailand?
Yes, whatever your passport. You first get a Non-Immigrant B visa from a Thai embassy or consulate, then a work permit is issued through the Ministry of Labour once you are set up with the employer. Some occupations are restricted for foreigners, so confirm your role qualifies. Start early.
Does the type of yacht matter?
On a bareboat charter (up to 12 guests who charter the yacht and hire a skipper) the rules above apply. On crewed charter, private yachts and superyachts, crew are often treated as seafarers and the route can differ with the flag and where the yacht is based, so confirm for that yacht. Working ashore follows ordinary work-permit rules.
Official source
Thai Ministry of Foreign Affairs (Non-Immigrant B visa) and the Ministry of Labour, Department of Employment (work permit). Confirm the current rules for your nationality before you travel.
What we ask of you
Keep your nationality correct in your profile.
If you hold a work permit for Thailand, tick the work-permit box.
Tell us early if your right to work is not sorted, so we can help before your dates.
Important
Immigration rules change and vary by nationality. This is general orientation, not legal advice, and Quarterdeck is not an immigration adviser. Confirm your own position with the official source above before you travel. We will not place you in Thailand if you do not hold the right to work. See also: Visas & work permits, overview.