Well... personally I believe the evidence which suggests that they're instinctive animals and have a *bit* of an issue with the idea that a snake thinks or feels in the context that WE use the word... Stimulus prompts response, response is instinctual and genetic... But that's as much of that debate as I'm going to put here on this thread and this topic unless someone wants to hear more... It's simply important to have a brief disclaimer.
There is a degree to which someone experienced with animals can "read" them and understand the likely response. Body language, the way muscles tense or relax, vocalizations, activity that's not normal... All are extremely valuable and can be used to deduce what state an animal is in and what their next likely behavior will be given continued application of identical stimulus... But that only goes so far. It's important to avoid anthropomorphism if you want a really clear idea of reptile behavior but that doesn't mean a human is totally incapable of understanding what's going on in that little greasy smear our favorite animals have for a brain. They are alien to our experience in many respects though, we can sympathize but not empathize with the fashion in which neurons fire off and behaviors are determined, this means that we can't know exactly what they might be thinking at all times, we can at best hazard an educated guess when the behaviors make it more obvious. Not many will argue that a snake which starts wildly thrashing, deficates and tries to swarm up the side of the tub is trying to be anyplace but where it is... How about one that doesn't actively swim but simply moves as much of the body out of the water as possible... or "treads" water in place... or sinks to the bottom and blows bubbles out it's nose? I know I can't determine exactly what's going on inside their head when the placement is forced to start and I'd like to think I've got a pretty good grasp of the more obvious body language.
"Genes, Like Leibnitz's monads, have no windows; the higher properties of life are emergent... And once assembled, organisms have no windows." - Edward Wilson, Sociobiology