If you're planning diving trips from Bangkok, rent gear here instead of on the islands. Sea Dragon Dive Center on Khao San has full gear rental for ฿800/day vs ฿1200+ on Koh Tao. They also do PADI certifications if you want to get certified before your island time. Gear is well-maintained and they'll do equipment checks before you travel. Saves money and guarantees properly fitting equipment.