Map from Google - Quirimbas

The climate of the Quirimbas (or Fortunate Islands), archipelago in the north of Mozambique, is tropical, hot all year round, with a warmer, more humid and rainy season from December to April, in which the maximum temperatures hover around 31/32 °C (88/90 °F), and a dry season from May to November, which includes a cooler period from June to August. In this relatively cool period, the southeast trade winds prevail, blowing steadily from the sea, while the warmest period is dominated by masses of warm, moist air coming from the interior of Africa.
Here are the average temperatures.
Average temperatures - Quirimbas
QuirimbasJanFebMarAprMayJunJulAugSepOctNovDec
QuirimbasJFMAMJJASOND
Min (°C)242423232119192020222324
Max (°C)313231313029282930313132
Min (°F)757573737066666868727375
Max (°F)889088888684828486888890

In the Quirimbas islands, nature is untouched, and they are rich in land and marine species of great interest, such as dugongs, dolphins, sea turtles, whales, sharks, in addition to the reef. The main islands are Metundo, Vamizi, Matemo, Sencar, Mefunvo, Quisiva, Ibo, Medjumbe, Quirimba, Rolas, and Quilaluia.

Rainfall is not particularly abundant, given that it amounts to about 850 millimetres (33.5 inches) per year; however, it exceeds 100 mm (4 in) per month from December to April, with a peak in March of even 200 mm (8 in), while it becomes scarce from May to November, when there are rare showers, most likely in May and November, that is the beginning and the end of the dry season.
Here is the average precipitation.
Average precipitation - Quirimbas
QuirimbasJanFebMarAprMayJunJulAugSepOctNovDecYear
QuirimbasJFMAMJJASONDY
Prec. (mm)145155200120301510821040125860
Prec. (in)5.76.17.94.71.20.60.40.30.10.41.64.933.9
Days11101393221114865

From November to mid-May, the archipelago can be affected by the tropical cyclones of the South Indian Ocean, although they are more likely from January to March.

The amount of sunshine in the Quirimbas is good from May to November, but it's still acceptable even in the wettest months, reflecting the fact that the rains come in the form of downpour or thunderstorm, which do not last long and soon give way to the sun.

Quirimbas

For swimming, the sea is warm all year round: the water temperature ranges from 25 °C (77 °F) in August, to 29 °C (84 °F) in the period from January to April.
Here are the average sea temperatures.
Sea temperature - Quirimbas
QuirimbasJanFebMarAprMayJunJulAugSepOctNovDec
QuirimbasJFMAMJJASOND
Sea (°C)292929292827262526272828
Sea (°F)848484848281797779818282

When to go

The best time to go to the Quirimbas runs from mid-May to October. Usually the weather is good also in the first half of May and November, but there is a risk, albeit not high, of the passage of a tropical cyclone. From mid-May to August, you can bring a sweatshirt for cooler evenings.