Thailand's eastern seaboard is probably not as well known or popular as the southern areas - for diving, but there is still plenty to see and do. First, however, we probably need to address a geographical issue: Thais refer to this area as the "Eastern Seaboard" because it is east of Bangkok. The coastline actually faces roughly southwest. To find east-facing beaches, you have to go west of Bangkok. Confused? Welcome to Thailand!
Closest to Bangkok, and its international airport, is Pattaya. The Vietnam-war era "R & R" destination has been trying to throw off its sleazy reputation, with only limited success. While the diving around Pattaya is not the best to be found in Thailand, it does seem to be the best place for wreck diving, if you're into that, or want to be.
Further along the coast, almost to the Cambodian border in fact, is Koh Chang. Thailand's second largest island has only recently been developing as a major tourist destination. There are some good dive spots scattered around the tiny islands between Koh Chang and Koh Kut.