The diving season in Pattaya is typically from November to April. Weather is typically cooler and calmer leading to improved sea conditions and better visibility (up to 20m). This coincides with the high tourist season so hotels and flights may be a bit more expensive.
Diving in Pattaya can be undertaken all year round however in the low tourist season (May to Oct) weather conditions are not typically great for diving. The seasonal monsoons bring wind and rain which ultimately means increased waves and poor visibility. You may be able to get a good deal on flights and hotels during this season but your dives may be cancelled or impacted. Just factor this in when deciding to go.