Map from Google - Bali

Bali is an Indonesian island, located just a few degrees south of the equator, and has a tropical climate, hot all year round, with a rainy season from November to March, and a relatively dry season from April to October, when, however, some showers are still possible. The tropical rains fall in the form of showers or thunderstorms, maybe intense, but usually short-lived, therefore after a short time they give way to the sun, even in the rainy season.
Temperatures are high and uniform throughout the year. They are slightly higher from October to March, when the daily average is around 27 °C (81 °F), and a bit lower from June to September, when the daily average is around 26 °C (79 °F). In the rainy season, high relative humidity makes the heat muggy.
Here are the average temperatures of the airport, located near the capital Denpasar.
Average temperatures - Denpasar
DenpasarJanFebMarAprMayJunJulAugSepOctNovDec
DenpasarJFMAMJJASOND
Min (°C)232323232322222222232323
Max (°C)313131313130292930313131
Min (°F)737373737372727272737373
Max (°F)888888888886848486888888

Throughout the year, 1,700 millimetres (67 inches) of rain fall in the southern part of the island, with a maximum from December to February, when the rainfall amount goes from 280 to 350 mm (11 to 13.5 in) per month, while it drops below 100 mm (4 in) per month from April to October, with a minimum in August of just 15 mm (0.6 in).
The rainfall amount, however, can change from one year to another, so if we analyze the rainfall records, we can see that sometimes the rainy season can last until April, or begin as early as in October.
Here is the average precipitation in Denpasar.
Average precipitation - Denpasar
DenpasarJanFebMarAprMayJunJulAugSepOctNovDecYear
DenpasarJFMAMJJASONDY
Prec. (mm)350280215907570501540901502901715
Prec. (in)13.8118.53.532.820.61.63.55.911.467.5
Days191820128743361016126

The wettest area of the island is the interior, occupied by hills and mountains, whose highest peak is the volcano Agung, 3,000 metres (9,800 feet) high; the northern coast is rainier than the southern one, being closer to the inland elevations. The southern part of the island, where we find the capital Denpasar, as well as cities such as Kuta, Seminyak and Pecatu, is therefore the most arid. If you are planning some hiking at high altitudes, keep in mind that above 1,000 metres (3,300 feet) it's cool in the evening and at night, especially in the dry season (see Bedugul, Kintamani), while above 2,000 metres (6,500 feet) it can get quite cold; besides, at noon and in the afternoon the chance of thunderstorms in the highlands is higher than in the southern side, where the majority of the resorts are found.
The sunshine amount in Bali is very good in the dry season, and remains acceptable even in the rainy season, because as we said the tropical rains, however intense, do not generally last long. On the contrary, in the interior a blanket of clouds often lies over the mountain peaks.
Sunshine - Denpasar
DenpasarJanFebMarAprMayJunJulAugSepOctNovDec
DenpasarJFMAMJJASOND
Sun (hours)667891010109977

From November to mid-May, but more often from January to mid-April, Bali can be brushed by tropical cyclones, which usually do not hit it directly, but pass not far to the south, bringing an increase in wind and rainfall.

Bali

For swimming, the sea in Bali is warm throughout the year: the water temperature goes from 29 °C (84 °F) from November to May, to 27 °C (81 °F) in July, August and September. For surfing, you'll prefer the western part of the Bukit peninsula, because very high waves form there.
Sea temperature - Bali
BaliJanFebMarAprMayJunJulAugSepOctNovDec
BaliJFMAMJJASOND
Sea (°C)292929292928272727282929
Sea (°F)848484848482818181828484

When to go

The best time to travel to Bali and enjoy its beaches, goes from June to September: the sun shines, the heat is not too intense, and in any case it's tempered by the breeze; there are just some short downpours or thunderstorms every now and then, however, more likely at night and in the early morning. August is the best month, being the driest. May and October too are months you may want to consider, being the best ones among those that do not fall in the high season: they are a bit more muggy and rainy, but they are still out of the rainy season proper; however, in inland and mountainous areas thunderstorms are frequent.

What to pack

All year round, light clothes, of natural fibres, sun hat, scarf for the breeze, a light sweatshirt for the evening. From November to March, light raincoat or umbrella.
For the reef, equipment for snorkeling, water shoes or rubber soled shoes.
For mountain climbing, hiking shoes, sweatshirt or sweater, jacket, raincoat.