As a general rule, North and Northeast Brazilian seawater (involving major travel destinations such as Fortaleza and all the Ceará region; Recife, Natal, Maceió and all the Pernambuco region; Sao Luís, Belém and the Amazon’s mouth region; Salvador and the Bahia region) is warm – or very warm, if you want.
North and Northeast coastal spots have typical tropical and subtropical sea water temps: above 80ºF (27ºC) all over the year. Winter and summer sea water temperatures vary very little: 2-3 degrees, which is not significant for beachgoers.
Here are some examples:
Belém seawater temperatures: 83ºF/28ºC (summer vs. winter variation: 2º maximum)
Sao Luis seawater temperatures: same as Belém
Fortaleza seawater temperatures: 81ºF/27ºC (maximum variation all over the year: 2ºF)
Natal water temps: 82ºF/28ºC, with almost no variation over the months.
Salvador da Bahia seawater temps: 83ºF/28ºC maximum (March-April); 81ºF/27ºC minimum (August)
Maceió seawater temps: same as Salvador da Bahia.
Recife seawater temperatures: 84ºF/29ºC maximum, April-May; 80F/26.5º minimum (August)
Bottom line: North and Northeast Brazilian tourist destinations have typical tropical seawater temperatures. It may exist slight variations depending on local and regional conditions – water flows from nearby rivers, for instance – but it doesn’t count much.
The time of the year, and local conditions, only become important in Rio de Janeiro area, and most of all in the far south travel destinations. South Brazil seawater temperatures are an entirely different matter, and you should consider the time of the year – if you want to enjoy beach life.
Rio de Janeiro seawater temperatures
The issue of seawater temperature in Rio de Janeiro and nearby travel spots such as Buzios or Angra dos Reis is a bit different. Seawater temperatures are still relatively warm all over the year, but not as warm as in other northern travel destinations; and there is a more significant difference between summer and winter water temperatures (though the winter temperatures may still be pleasant for those who do not require warm-tropical water temperatures).
Here are some average figures for seawater temperatures, in Rio de Janeiro:
October to May/June seawater temperatures (high season; months with the warmest water temperatures): 77ºF/25ºC
July, August, Septembe (brazilian winter; months with the coldest water temperatures): around 71ºF/22ºC
Other months: sea water temperatures between the values mentioned above.
Sao Paulo region seawater temperatures
Seawater temperatures in Santos and nearby beaches (the coastal area of Sao Paulo) are not very different from those in Rio de Janeiro. Just 2 or 3 degrees lower.
Sao Paulo – Santos – Seawater temperatures:
January to April seawater temperatures: 80ºF/26ºC
May, June and October to December: around or above 73ºF/23ºC
South Brazil seawater temperatures: Florianopolis, Curitiba and Porto Alegre
Water temperatures in Florianopolis and Curitiba (in the South) are a lot colder, during the wintertime (June to September).
Florianopolis seawater temperatures (average):
July to September: 65ºF/19ºC
January to March: 78ºF/25,5ºC
Other months: seawater temperatures between the ranges mentioned above.
Curitiba (Paranaguá) seawater temperatures (average):
About one degree less than those of Florianopolis.
To the south, seawater temperatures drop still more. Porto Alegre, in the far south of Brazil, has a significantly different seawater temperature pattern: average temperatures can drop to 60ºF/16ºC in the winter months (June to September), while in the summertime they average 75ºF/24ºC.
Inmet.gov.br, Instituto Nacional de Meteorologia