Map from Google - Galapagos

The Galápagos Islands, located about a thousand kilometres (600 miles) west of Ecuador, have a tropical and semi-arid climate, with a relatively warm and rainy season from January to May, and a cool and dry season, but also cloudy and misty, from July to November.
The landscapes of the Galápagos are barren, except in the highlands of the larger islands, which receive more abundant rainfall. As was already noted by Charles Darwin, who as we know stayed there studying the peculiarities of the species living in the islands, their climate is cooler than one would expect from a place located near the Equator, because of the Humboldt Current, which reaches the area after flowing in the sea west of South America. Anyway, here the climate is variable from one year to the other, because there are different ocean currents that meet or take turns in these parts (there is also a warm current coming from Central America, which runs at no great distance and is more active in the years of El Niño), therefore the weather is difficult to predict.

Sea currents around the Galápagos Islands
Here are the average temperatures in Puerto Baquerizo Moreno.
Average temperatures - Puerto Baquerizo
Puerto BaquerizoJanFebMarAprMayJunJulAugSepOctNovDec
Puerto BaquerizoJFMAMJJASOND
Min (°C)232323232221191918192021
Max (°C)293030302927252424252627
Min (°F)737373737270666664666870
Max (°F)848686868481777575777981

As mentioned, in these islands there are two seasons: the warm season from January to May, with maximum temperatures around 29/30 °C (84/86 °F), and a relatively cool season from July to November, called Garúa, with highs around 24/25 °C (75/77 °F), when however lows remain acceptable, around 18/19 °C (64/66 °F), and when the sky is often covered by a mist, which causes the condensation of tiny droplets (called garúa from which the season takes its name) and by low clouds (due to the thermal inversion generated by the cool ocean currents). This period is the least rainy of the year in coasts and plains, while on inland hills and mountains, there can be some rains. The highest peak is the Volcán Wolf, 1,707 metres (5,600 feet) high, situated on Isabela Island.
The warm season, from January to May, is instead the rainiest period, but normally the rains are not abundant, and in any event they occur in the form of afternoon showers that do not overshadow too much the sun. The rainiest month is March.
Here is the average precipitation. We can notice the fact that in the dry season, few millimetres per month accumulate, due mostly to drizzle and dew formation.
Average precipitation - Puerto Baquerizo
Puerto BaquerizoJanFebMarAprMayJunJulAugSepOctNovDecYear
Puerto BaquerizoJFMAMJJASONDY
Prec. (mm)50658535162457757288
Prec.(in)22.63.31.40.60.10.20.20.30.30.20.311.3
Days965431346641061

However, tourists flock to the beaches during the rainy period, because in addition to being the sunniest, it's the one in which the sea is the warmest.
The rainfall amounts to less than 500 millimetres (20 inches) per year on the coasts, so it's not abundant; however, it's irregular, and can become abundant in the years of El Niño, and definitely poor in the years of La Niña. During the most intense El Niño years, such as 1982-83 and 1997-98, the climate of these islands becomes full tropical, with abundant rainfall.

Galápagos

The sea is warm enough for swimming from January to May, while it gets a little cool from July to November, when the temperature drops to around 21/22 °C (70/72 °F).
Sea temperature - Galápagos
GalápagosJanFebMarAprMayJunJulAugSepOctNovDec
GalápagosJFMAMJJASOND
Sea (°C)242626262523222121222223
Sea (°F)757979797773727070727273

When to go

The best time to visit the Galápagos, especially for a beach holiday, runs from January to May, because it is the warmest and sunniest, although there may be some downpours or thunderstorms in the afternoon. Nothing prevents you from visiting these islands during the Northern Hemisphere summer, maybe to explore nature, since it almost never rains in the plains and the temperature is pleasant, even though you have to take into account mists, haze and cloudy skies.

What to pack

From December to May (warm period): light clothing, a light sweatshirt for the evening, light raincoat or umbrella for the showers. In the inland hills and the Volcán Wolf, a sweatshirt and a raincoat maybe a bit heavier, hiking shoes.
From June to November (cool period): light clothes, sweatshirt or sweater and light jacket for the evening; for hiking in inland hills and the Volcán Wolf, hiking boots, sweatshirt and raincoat.

For the reef, equipment for snorkeling, water shoes or rubber soled shoes.