While Thailand is a country that attracts a large number of foreigners who love to sunbathe, the culture there is still rather conservative. Keep this point in mind when choosing your bathing suit for the holiday. Avoid wearing skimpy swimsuits if you can help it. Of course, if you are staying in a private apartment you will be able to wear what you prefer. But if you stay at a resort that is located in close proximity to local settlements, you will have to wear decent bathing suits. This really is not a rule and no one will ever come up to you and condemn your choice of attire. But it's always nice to be respectful of the culture of the land that you visit.
Having a pretty beach cover-up will also be quite useful when walking around the beach. You should, however, refrain from wearing the cover-up when visiting restaurants or stores as that will rarely be deemed appropriate.