There was a study on the Amazon river temps and related or nearby rivers over a span of 3 weeks at various depths. (I can't vouch for all of the locations as this was a 1963 study and names may have changed. In addition, some of these are apparently estuaries, so may be "local" names.)
The following rivers and locations were found to have nearly identical water temperatures all day long at depths of surface, zero feet (surface), 10 feet and 50 feet of 82.5-86.0 F. In short the range is very small 24 hours per day at any depth of 50 feet or less within these locations.
Baia de Guajara
Rio Para
Estraito de Boiuca
Furo do Tajapuru
Furo do Ituquara
Amazon River (south channel)
Rio Tapajos
Rio Negro
Parana do Careiro
Rio Solimoes
Rio Madeira
The temperature of the rain forest does of course have a wide range of as much as 30 degrees F from day to night. And it's certainly plausible that shallow, slow moving areas might get cooler than 80F during the night. But it's hard to imagine where fish (at least from these areas) are finding cooler water.
For those that want to grab the .pdf, it's here.
http://pubs.usgs.gov/circ/1964/0486/report.pdf