The Similan and Surin Islands are some of Thailand's most popular dive destinations. Most of the visitors to these islands do so on liveaboard dive boats, but it is possible to catch a speedboat and dive the same sites while staying in mainland Thailand.
https://www.mymola.com/diving-thailand-s-similan-and-surin-islands-without-a-liveaboard/
https://www.mymola.com/diving-thailand-s-similan-and-surin-islands-without-a-liveaboard/