Map from Google - Nicaragua

Nicaragua has a tropical climate, hot all year round, with average daily temperatures ranging between 25 and 27 degrees Celsius (77 and 81 °F) on the eastern side, and between 26 and 28 °C (79 and 82 °F) in the west. The coolest period is from November to January, while the warmest months are April and May. The real difference between the seasons is found in the rain: there are a rainy and a dry season, the latter is longer and more marked on the western slopes and in inland plains and valleys, while it's almost non-existent on the eastern side, which is exposed to the north-east trade winds, and therefore receives rainfall even in winter, so that its climate can be defined as equatorial.
Consequently, rainfall is abundant on the east coast, where it even exceeds 4,000 millimetres (160 inches) per year, while it's much less abundant in the central plains (where the capital Managua and the lakes Nicaragua and Managua are located) and along the Pacific coast.
In Managua, 1,150 mm (45 in) of rain per year fall, and the dry season, which runs from December to April, is very pronounced, so much so that it almost never rains. In the rainy season, from May to mid-November, the wettest months are June and October, with more than 200 mm (8 in) of rain per month.
Here is the average precipitation.
Average precipitation - Managua
Prec. (mm)4156753001351301802456061147

In the period from June to November, the maximum temperatures are around 30/31 °C (86/88 °F) and the humidity is high; from February to May they are around 32/33 °C (90/91 °F), but they can quite often reach 35 °C (95 °F): although the humidity is lower, it's still sufficient to give a feeling of sultriness. It goes a bit better on the Pacific coast, where the temperatures are a bit lower and the breezes blow.
Here are the average temperatures.
Average temperatures - Managua
Min (°C)212122232423232323222221
Max (°C)313233333331303130303130
Min (°F)707072737573737373727270
Max (°F)889091919188868886868886

In Bluefields, on the Caribbean coast, precipitation amounts to 4,000 mm (160 in) per year; it rains a lot and almost every day from May to December, with a peak in July, with as high as 665 mm (26 in) of rain. March and April are the only months in which the rains give a little break, dropping to around 80 mm (3 in) per month, in both cases in 12 days.
Here is the average precipitation in Bluefields.
Average precipitation - Bluefields
Prec. (mm)26513080753455006655453103453904004050

It goes a bit better further to the north, in Puerto Cabezas, where 3,000 mm (120 in) of rain per year fall. So here and in general along the Caribbean coast, there is no really dry month; due to the rains and the trade winds, the temperatures are a bit lower than in Managua, but relative humidity is consistently high.
Here are the average temperatures in Bluefields.
Average temperatures - Bluefields
Min (°C)222222232423232323222222
Max (°C)282930313130292929292928
Min (°F)727272737573737373727272
Max (°F)828486888886848484848482

The sea is warm all year round, in both coasts, as we can see from the water temperature at Bluefields.
Sea temperature - Bluefields
Sea (°C)272727282828282829292828
Sea (°F)818181828282828284848282

The amount of sunshine on the Caribbean coast of Nicaragua is never great, it's acceptable in March and April (with 7 hours of sunshine per day) and is mediocre from June to August (with 3/4 hours per day). It goes better on the west side: in Managua there are 8 hours of sunshine per day in February and March, and 7 in November, December, January and April. The least sunny month (and as we have seen also the rainiest) in the capital is June, with only 4 hours of sunshine per day.

Lake Nicaragua

Nicaragua is known as the land of lakes and volcanoes. Some volcanoes rise from the plains, such as San Cristobal, other from lakes, such as Concepción, which emerges from Lake Nicaragua. In the north-central inner region, there is a mountain range, called Cordillera Chontaleña: in the highlands the weather becomes mild above 1,000 metres (3,300 feet), in the so called tierras templadas.
Nicaragua is located in the path of hurricanes, which may hit the country in the period from June to November, coming from the east, ie the Caribbean Sea, but are most likely from August to October. A particularly intense hurricane was Joan-Miriam, which caused serious damage throughout the country, and in particular to Bluefields, in late October 1988.

When to go

The best time to visit Nicaragua is from December to April in the central and western part, while it's limited to March and April along the east coast, due to the rains.

What to pack

For the tierras templadas and the slopes of volcanoes: all year round, light clothing, a sweatshirt or sweater for the evening; raincoat or umbrella.
For plains and coasts: light clothing, a sweater or sweatshirt for the evening from November to March; in the rainy season (May to mid-November in Managua, even throughout the year on the east coast), a light raincoat for showers and thunderstorms.