In my opinion, and based on my own experiences the temps you are experiencing right now will not cause you any problems.
I have kept a group of dwarf caiman outside yearound here in Texas with a little supplemental heating of the water. The big thing alot of people don't realize when talking about temps and crocs is that some species of crocs will 'survive' pretty cold temperatures IF a few other conditions are met.
Crocs tend to lose motor functions and hence swimming ability as temps drop. A croc can survive a colder temp IF the water is shallow enough to prevent drowning. Once the temp drops too low and motor functions prevent the croc from getting to the surface it's down to the bottom. They'll drown just like you will.
Much lower than the temps you are experiencing at night and I'd heat the water. The days are fine.
The Ultimate Guide to crocs
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CDieter
'Reason, observation, and experience; the holy trinity of science.'