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 center-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 50 to 70 inches 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 80 inches 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, 5,900 feet 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 200 inches 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 86 °F in November and December, it begins to rise slightly already in January, and it reaches its highest values in April, with 95 °F, but with peaks of 105 °F; then it decreases slightly during the long rainy months.
Here are the average temperatures of Phnom Penh.
Average temperatures - Phnom Penh
Phnom PenhJanFebMarAprMayJunJulAugSepOctNovDec
Min (°F)727375777777777775757372
Max (°F)909195959393919190888686

Throughout the year, 55 in of rain fall, with a minimum in January, when it almost never rains, and a maximum in October, with 10 in 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. (in)

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 (°F)707273777775737573737372
Max (°F)888890909088888686888686

Here the rains are more abundant, given that they reach almost 80 inches per year; in addition, the rainiest months are July and August, with more than 12 inches per month. Here is the average precipitation.
Average precipitation - Kampot
Prec. (in)

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 (°F)828284868686848484848482

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 fiber, 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.