Map from Google - Cambodia

In Cambodia, the climate is tropical, hot all year round, with a rainy season from May to mid-November, when the south-west monsoon prevails, and a dry season from mid-November to April. The dry season can be divided into two periods: the first, is the least hot of the year, while the second, from mid-February to May, before the monsoon arrives, is the hottest of the year. The monsoon withdraws first in the north, in early November, and then in the centre-south, between the middle and the end of the month.
The climate of Cambodia is fairly uniform, because the country is not large. In a typical year, from 1,300 to 1,800 millimetres of rain fall in inland areas; the rainiest region is the south-west, towards the coast of the Gulf of Thailand, where the rainfall exceeds 2,000 mm per year; here there are vast areas covered by forest. The climate becomes a bit cooler in the highlands, such as the Cardamon Mountains, which reach their highest point in Phnom Aural, 1,800 metres high, whose slopes are covered by an impenetrable forest and are protected by a nature reserve. Mountainous areas, in addition to being the coolest, are also the rainiest: they receive even 5,000 millimetres per year of rain per year, and may experience some showers in the afternoon even before the monsoon season (February to April).
The coolest month in Cambodia is December, while the hottest months are April and May, when the heat is oppressive; in the rainy season the temperature is a bit lower, but the humidity is higher, so the weather is hot and muggy.
In the capital, Phnom Penh, the daytime temperature is 30 °C in November and December, it begins to rise slightly already in January, and it reaches its highest values in April, with 35 °C, but with peaks of 40 °C; then it decreases slightly during the long rainy months.
Here are the average temperatures of Phnom Penh.
Average temperatures - Phnom Penh
Phnom PenhJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)222324252525252524242322
Max (°C)323335353434333332313030

Throughout the year, 1,400 mm of rain fall, with a minimum in January, when it almost never rains, and a maximum in October, with 250 mm of rain. The end of the monsoon, therefore, is the wettest period, also because of heavy rains brought by typhoons; the greatest risk of flooding is found in the area crossed by the great river, the Mekong.
Here is the average precipitation.
Average precipitation - Phnom Penh
Phnom PenhJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec. (mm)7104075135155170160225255125451402

Cambodia can actually be hit by typhoons, which come from the east and generally are weakened when they pass over the country, because they discharge the bulk of their energy over the coasts of Vietnam, however, they can still bring heavy rains, which can cause flooding. Typhoons occur between May and November, but especially between September and November; sometimes they can occur later, in the first half of December.
The climate of the ancient capital, Angkor Wat, located in the northwestern part of the country, is similar to that of Phnom Penh, but lying in the northern part, here the monsoon retreats a bit sooner, on early November; besides the winter temperature is slightly lower.
Along the coast, the pre-monsoon heating is less pronounced, due to the thermal inertia of the sea, as we can see from the temperatures of Krong Kampot.
Average temperatures - Kampot
Min (°C)212223252524232423232322
Max (°C)313132323231313030313030

Here the rains are more abundant, given that they reach almost 2,000 mm per year; in addition, the rainiest months are July and August, with more than 300 mm per month. Here is the average precipitation.
Average precipitation - Kampot
Prec. (mm)102080100190210320330250230140401920

The amount of sunshine in Cambodia is good from December to February, while it's barely sufficient if not low from June to October, because of the cloudiness due to the monsoon. In Phnom Penh, there are on average 8 hours of sunshine per day in January, 9 in March and only 5 July.

Angkor Wat

For swimming, the sea is warm all year round, as we can see in the following table.
Sea temperature - Koh Kong
Koh KongJanFebMarAprMayJunJulAugSepOctNovDec
Sea (°C)282829303030292929292928

When to go

The best time to visit Cambodia goes from December to mid-February, being the coolest, and also the driest and sunniest of the year. If you want you can avoid the first half of December, in which sometimes a latecomer typhoon has passed over the country.
For swimming, the best time is still from December to February, even though on the coast the increase in temperature in the period from March to May is less pronounced than in the interior.

What to pack

All year round, lightweight clothing of natural fibres, comfortable shoes, scarf for the breeze on the coast; from December to March, a sweatshirt for the evening; in the rainy season, light raincoat or umbrella. For the mountains, spring/autumn clothes, raincoat or umbrella, sweater and jacket for the evening, a bit warmer from December to March.
To enter temples, it is customary to remove shoes, dress neatly and cover a little.