In April you can dive both the Gulf Of Thailand (Koh Tao) and the west coast of Thailand (Andaman Sea). It sorta depends on how much time you have, what your budget is, and if it's a dive focused holiday or you want to mix in some temples, hiking, and cooking demonstrations. Koh Tao wins if cost is the main factor as the diving is relatively inexpensive and the diving is decent...some sites are actually pretty good if the conditions are good and the marine life cooperates. There are maybe a couple dozen dive shops and many bungalows, hotels, and resorts. It's a quick overnight hop from Bangkok to get there by bus or train then ferry. You can also fly to Koh Samui and catch the ferry to Tao. There are many detailed reports here and YT on Koh Tao...be advised it is focused towards a young crowd of divers, dive students, and partygoers.
Koh Phuket is a good all around location if you want to day dive from boats or even shore dive (Kata Beach). A bit more expensive than Koh Tao but the diving is better. There are also many topside activities for non-diving days or non-divers. It's easy to get to with direct flights from Bangkok and many international gateways. Krabi on the west coast mainland is an option, with local trips to near-in islands and sites and longer day trips to Koh Phi-Phi. Koh Lanta, just south of Krabi is the jumping off point for day trips to some local islands and wrecks, Koh Phi-Phi, and the Koh Haa National Marine Park (including Hin Daeng/Muang...two of the top underwater pinnacles in Thailand). Finally, if you fancy a Robinson Crusoe getaway to a remote island with IMO the best diving in Thailand, head to Koh Lipe, in Trang Province, all the way down the west coast near the Malaysian border. This is a tiny island (can walk from end to end in 20 minutes) that sits in the Tarutao National Marine Park. Exceptional diving with sites just 5-20 minutes by boat offshore, and further sites up to 45 minutes away. There is everything here from exceptional macro, extensive coral reefs, to schools of barracuda, jacks, and sometimes whale sharks.