I don't really think that there was any sort of thrall involved here. I think that, basically, Buffy realized for the first time that, no matter how powerful she might be, and you must admit that for a 15 year old girl to suddenly wake up and find that she's stronger than her school's entire defensive line would be pretty heady, there were things out there far stronger than she was, and that she very well might die fighting them. We're talking about a young girl who didn't even have her driver's lisence, and suddenly she hears from her mentor and father substitute that she HAS to fight the Master, and that he WILL kill her. She was scared to death, quite literally. She was like the deer, frozen in terror, when the lion attacks. The fact that she managed to discharge one single crossbow bolt before letting that fear completely immobilize her is a miracle in itself. Combine that with the fact that, for some unexplained reason, the Master neither drained nor turned her, AND that her friends arrived before she drowned takes it into the realm of the phenomenal.
We're not talking about the seasoned fighter that Buffy was to become within the next few years here, we're talking about a young, frightened, relatively inexperienced girl. That she survived at all is certain proof that her guardian angel, the real one, not the broody one, was taking special interest in her welfare.