> If sentience is the emotional/feeling part of our brain, what exactly does it mean for a human, or an animal, or a fish, to feel? And what would the analogue for that be in a machine?

If you switch to the reinforcement learning paradigm, then the "value function" might be analogous to emotion. It assigns a value (good or bad) to each perception, state, situation or context in order to decide what is the best action to take.