Michaelis definitions of restinga: 1. sandbank, shoal. 2. salt marsh. 3. reef, shelf. 4. long spit of land running into the sea. 5. small tract of wood bordering a river.

Since the word has so many meanings in Portuguese, I think it's wiser to make a choice of one of the known English translations. In your case, it appears to be "salt marsh" because the sandbank/shoal and the reef/shelf are under water, and definitions 4 and 5 wouldn't appear to apply.

A salt marsh is not a geographical feature unique to Brazil.

The following can be ruled out in your context:

sandbank:

a submerged bank of sand near a shore or in a river; can be exposed at low tide
wordnet.princeton.edu/perl/webwn

shoal:

# a sandbank in a stretch of water that is visible at low tide
# a stretch of shallow water
# shallow: make shallow; "The silt shallowed the canal"
wordnet.princeton.edu/perl/webwn

# A sandbank or sandbar that makes the water shallow; specifically : an elevation which is not rocky and on which there is a depth of water of six fathoms (11 meters) or less
response.restoration.noaa.gov/cpr/watershed/calcasieu/calc_html/resources/glossary.html

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Note added at 22 hrs (2006-12-08 22:14:03 GMT)
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I really apprecaite Andrea providing the official definition, but it doesn't seem to differ very much from the definitions of "salt marsh."

salt marsh

A community of organisms dominated by plants that are tolerant of wet, saline soils, generally found in low-lying coastal habitats which are periodically wet and unusually saline to hypersaline. The term salt marsh summarizes the saline conditions of the habitat as well as the emergent vegetation which dominates it. plants which grow in salt marshes are thus tolerant of two conditions: saline and wet.

salt marsh n. Low coastal grassland frequently overflowed by the tide.
www.answers.com/topic/marsh