Cold Blood.

Are Nagas and Mermaids coldblooded? I know that snakes are, and same with fish, but both giants have human halves. Although, being cold blooded would explain why they don't need to eat as much as you might expect from a 160 foot tall carnivore. A downside of being cold blooded, is that the predator would not be able to regulate their own temperature, and would be confined to warmish areas, incapable of traveling in either arctic or desert terrain. Unless magic explains it. Perhaps I am over analyzing this.

The reason I ask, is that being cold blooded would effect thermal cameras, and make heat sensitive goggles an incomplete method of predator tracking. Bad luck humans.

I wouldn't think so

main reason is that if Ice nagas were cold blooded, they'd be forever in hibernation mode 

another reason is that water tends to be cold, especially the deeper you go so the mermaids would probably do the same. 

I think the thing for mermaids is that they have adapted to the temperatures of water

I haven't heard any official word, but judging from the stories I've seen a lot of people seem to consider them cold-blooded. Now if I were designing Felarya I'd make them warm-blooded as it seems a little more realistic and practical.
It's possible that some species are and some aren't. Perhaps, for example, fire nagas and ice nagas are warm-blooded, but desert-dwelling nagas aren't.

Well, the leatherback turtle can have a range in cold water and dive down to at least 400m, because they have a limited ability to generate heat, though they're still considered cold-blooded.

Salmon sharks can be found far north, but they are homeothermic, but are not endothermic and tachymetabolic. Warm blood runs past cold blood, warming the cold blood in the process. The abyss can be as cold as 0.6 degrees, but fish still swim down there.

Thanks guys, that helps a lot.