Africa

In August, the Northern Hemisphere is in the middle of summer: in North Africa, in the Gulf of Guinea and south of the Sahara, the monsoon is still active; in the Red Sea it's still too hot. Along the coast of Morocco there are fogs and mists in the morning, and the sea is cool, but all in all you can go. In Cape Verde the rainy season begins, though irregular. In the Southern Hemisphere it's winter, and even at tropical latitudes it can be a little cool. In the Seychelles, in theory you can go, even though a strong wind often blows. In Mauritius it can be a little cool, and there can be a moderate wind and some rain.
Where to go: Azores, Canary Islands, Comoros, Kenya, Madagascar (north), Madeira, Morocco (Atlantic Ocean), Mayotte, Mozambique (north coast), Tanzania, Zanzibar

Asia

The monsoon season continues in much of south-central Asia, sparing some parts of Indonesia.
Where to go: Indonesia (Bali, Walea), East Timor

North and Central America

In the Gulf of Mexico and Central America the weather is hot, muggy and rainy, and there's the risk of hurricanes, higher than in July. In the coast of Southern California, the sea is still quite cold, but it goes a bit better than in July.
Where to go: United States (Los Angeles), Mexico (Baja California north)

Caribbean Sea

The weather is still hot and sultry, with heavy showers, and an increased risk of hurricanes compared with July. If you are lucky you may well experience a nice period, but generally this is not an advisable month. The driest islands, and the most sheltered from hurricanes, are the Leeward Antilles (Aruba, Bonaire, Curacao), as well as the islands of Venezuela.

Mediterranean Sea

It's the hottest month along with July, but the sea temperature is slightly higher. We can not exclude some thunderstorms in northern Italy, France, Slovenia and northern Croatia, and along the coast of the Black Sea of Bulgaria and Romania. The eastern Turkish coast of the Black Sea remains quite rainy even in summer. The southern coast of Turkey is sunny, but it can be a bit hot. You can also go to Portugal and Spain in the Atlantic Coast, where, however, the sea is a bit cool.
Where to go: Albania, Algeria, Bulgaria, Croatia, Cyprus, Egypt, France, Greece, Israel, Italy, Lebanon, Morocco, Malta, Montenegro, Romania, Slovenia, Spain, Tunisia, Turkey (west and south coast), Ukraine

South America

In the Southern Hemisphere it's winter. In the north of the continent, in the Northern Hemisphere, it's summer, so it's hot and rainy. In some areas around the Equator, the dry season begins.
Where to go: Brazil (Belem, Fortaleza)

Oceania

It's still winter in the Southern Hemisphere, which means that there is a wide choice, among the islands that lie at tropical latitudes (but not too far to the south), where you will find sunshine, little rain, and a pleasantly warm weather tempered by the breeze.
Where to go: Australia (Darwin), Cook, Fiji, Christmas Island, Nauru, Niue, Papua New Guinea (Port Moresby), French Polynesia (Tahiti, Bora Bora, Marquesas Islands), Solomon Islands, Samoa, American Samoa, Tokelau, Tonga (north-central), Tuvalu, Vanuatu (north), Wallis and Futuna