Hi Wanderer, Where in Thailand are you planning to visit? The most popular place and also considered the best cruising grounds will be Phuket. There's 3 marinas there now but only one that can handle yachts above the 100 feet range. The marinas are Phuket Boat Lagoon www.phuketboatlagoon.com. This is the most complete marina with restaurants, hotel, residential, chandlery, etc. etc. Down side is it can only take smaller yachts due to the shallow access. Second marina is Royal Phuket Marina which is only next door to the Phuket Boat Lagoon which means it's only for smaller yachts since they also share the same shallow channel as the Phuket Boat Lagoon. www.royalphuketmarina.com. The third marina is Yacht Haven on the Northeast side of Phuket and the only marina that can handle large yachts. It's a bit further away from town and not much around the marina itself. www.yacht-haven-marina.com. It's quite an amazing cruising ground around Phuket which by itself has many islands around for cruising. We also have one of the top ten dive sites in the world the Similan Islands. If you head North you will be in Burma with hundreds of islands basically unexplored. Head South and you have Malaysia and at the very Southern tip, Singapore. Heading West from Phuket you also have the Andaman Islands of India. It's a very dynamic and huge area for cruising which is basically unexplored. I hope this is of help to you. Do let me know if you need more information. Cheers.
I can only think of one marina in the Gulf of Thailand. It's called Ocean Marina in Pattaya. www.oceanmarinayachtclub.com There aren't many marinas on that side of Thailand and the cruising around the area isn't as nice either. Not that many islands around. Let me know if I can be of further help.