Africa
In March, in the Gulf of Guinea both temperature and humidity increase, therefore the first showers occur. On the coast of northern Tanzania, the season of the "long rains" begins.
Where to go: Guinea, Guinea-Bissau, Gambia, Djibouti, Senegal (south)
Asia
It starts to get hot, though there's still little rain, in the seaside resorts of India and Thailand.
Where to go: Philippines, Maldives, Malaysia (Kota Baharu, Kota Kinabalu), Oman, Thailand (Phuket, Ko Samui)
North and Central America
Sometimes, especially in the first part of the month, it can be a little cold in Florida (especially in the northern and central part), and a little cool in Riviera Maya. In Central America it's hot and sunny, and only the north side of Panama and Costa Rica remains rainy.
Where to go: Belize, Costa Rica (Pacific coast), El Salvador, Guatemala (both coasts), Honduras (both coasts), Mexico (Acapulco, Cancun, Riviera Maya), Nicaragua (both coasts), Panama (south coast), United States (Florida)
Caribbean Sea
It is usually a good month. Compared with February, the temperature rises slightly, but there is still the risk of a bit of cool weather, especially in the first part of the month, on the northernmost islands.
Where to go: Anguilla, Antigua and Barbuda, Barbados, British Virgin Islands, Cayman Islands, Colombia (Caribbean Sea), South-eastern Cuba (Marea del Portillo, Santiago de Cuba), Dominica, Dominican Republic (southern and eastern coast), Grenada, Guadeloupe, Jamaica, Leeward Antilles (Aruba, Bonaire, Curacao), Martinique, Montserrat, Puerto Rico, Saint Barthélemy, Saint Kitts and Nevis, Saint Lucia, Saint Martin/Sint Maarten, Saint Vincent and the Grenadines, Trinidad and Tobago, Turks and Caicos, U.S. Virgin Islands, Venezuela (islands).
South America
In the Southern Hemisphere it is still summer at tropical latitudes, therefore a hot, muggy and rainy weather predominates. Further south, however, autumn begins. We must therefore go beyond a certain latitude, but not too much.
Where to go: Brazil (Rio de Janeiro)
Oceania
In the Southern Hemisphere it is still summer, with sultry heat, heavy rains and the passage of cyclones. So it's better to look at the Northern Hemisphere, or in Australia, in Perth, where the sea is a bit cool, more or less as in February, but still acceptable.
Where to go: Australia (Perth), Guam, Marshall Islands, Northern Mariana Islands, Palau