Map from Google - Songkhla

In Songkhla (or Singgora), a city in southern Thailand, the climate is tropical, hot and humid all year round, with a rainy season, due to the monsoon, which lasts from May to January, but has a pronounced peak between October and December, ie in the period of the "retreating monsoon", while from June to September, and usually also in January, the rains are not too abundant. In the short dry season, from the middle or the end of January to April, some showers are still possible.
Precipitation amounts to about 87 inches per year. The rains are particularly abundant in the last months of the year, because at that time the monsoon comes from the northeast (retreating monsoon), affecting in full the coasts exposed to the northeast, therefore the eastern part of peninsular Thailand. Typically the worst month, with real torrential rains, is November, but sometimes it can happen even in October, December and the first part of January.
Here is the average precipitation.
Average precipitation - Songkhla
SongkhlaJanFebMarAprMayJunJulAugSepOctNovDecYear
SongkhlaJFMAMJJASONDY
Prec. (in)6.12.42.43.54.73.93.73.74.112.422.617.387
Days1477101412121314222320168

The temperature is high throughout the year, however, it's slightly lower from November to January, when highs hover around 84/86 °F; the heat is annoying due to humidity, also because behind the town there is a plain with some lakes (which are actually lagoons), which provide additional heat and moisture. From March to October, there can be hotter days, with peaks of 95/99 °F. Luckily, sea breezes blow.
Here are the average temperatures.
Average temperatures - Songkhla
SongkhlaJanFebMarAprMayJunJulAugSepOctNovDec
SongkhlaJFMAMJJASOND
Min (°F)737575757575757573737575
Max (°F)868890919191919190888684

The amount of sunshine in Songkhla is good from February to April, that is, in the driest period, but it's not scarce even in the other months, because the rains occur in the form of showers and downpours, which usually do not last long; only from October to early January, that is, in the wettest months, there can be more prolonged cloudy periods.

Songkhla, like other areas of Thailand, may be affected by tropical cyclones, from May to November, although the Gulf of Thailand is a relatively sheltered area, hence cyclones are quite rare and besides they are rarely intense.

Songkhla

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

When to go

The best time to visit Songkhla goes from late January to April, being normally the least rainy, with a preference for February, because it is, albeit slightly, the least hot. In January, depending on the year the rains may be more or less abundant, in particular in the first half, when at times they may still be torrential. In summer, in June, July and August, there may be long periods without rain and with an intense heat, with highs around 95 °F, while at other times there may be showers, more or less intense, almost every day; furthermore, as mentioned there is a risk, albeit not high, of the passage of tropical cyclones.