Most people don't realize you can do proper diving from Bangkok. Koh Samet has 10-20m visibility and decent coral at sites like Ao Thian. Several dive shops run day trips for ฿2800-3500 including transport from Bangkok.

Not world-class diving, but if you're certified and missing the ocean, it scratches the itch. Two dives plus lunch, back to Bangkok by 8 PM. Water temp is 27-29°C year round.