Map from Google - Pattaya

The climate of Pattaya is tropical, hot all year round, with a dry season from December to April, and a rainy season from May to mid-November, due to the southwest monsoon. The annual precipitation amounts to about 44 inches, so the rains are not that heavy, if not in the first phase (May) and the last (September-October): this is because the bulk of the rainfall is released on the Burmese west coast, which therefore protects this area of Thailand. However, some strong downpours can not be excluded also from June to August, although on average in this period, rainfall remains around 3.7/4.7 inches per month.
In March and April, before the monsoon arrives, there may be some increase in rainfall, although the sunshine amount is still good.
Here is the average precipitation.
Average precipitation - Pattaya
Prec. (in)

The period before the monsoon, from mid-February to May, is also the hottest of the year: the maximum temperatures can reach 93/95 °F, and the minimum temperatures around 82 °F. Luckily the breezes temper the heat. On the contrary, in winter, from December to mid-February, sometimes it can get a bit cool, so that lows can drop below 70 °F, and highs can stay at around 75/77 °F.
Here are the average temperatures.
Average temperatures - Pattaya
Min (°F)737577798181797977757372
Max (°F)888890919090888888888686

The amount of sunshine in Pattaya is good from December to April, the dry period, while it is not very good from May to September, in the monsoon period, when the sky may remain cloudy for several hours, although, after all, the sun shines often enough.

Pattaya can be reached by tropical cyclones, from May to November, but this area of Thailand is quite sheltered, because usually cyclones follow other paths. However we can not exclude that some cyclones pass over the area, bringing heavy rainfall, associated with floods, and wind, as happened with typhoon Nari in October 2013.

Pattaya, coastline

For swimming, the sea is warm all year round, as you can see in the following table.
Sea temperature - Pattaya
Sea (°F)828284868686848484848482

When to go

The best time to go to Pattaya goes from December to April, that is, in the dry season, with a preference for the period December to mid-February, because from mid-February to April there is an increase in temperature. From December to February you can bring a sweatshirt for any cool evenings.
If you want to go in summer, in July and August, you will have to cope with a sultry heat, increased cloudiness, some showers, sometimes strong, and the risk of a tropical storm or cyclone. But depending on year and period, even in these months the weather can be decent or good for weeks.