Anything that is treated lightly is basically allowed to continue and often made into a game by babies. With that in mind, sometimes I have to come down more firmly on something "small" if I really don't want it to continue and escalate, even though technically it's not always fun for me to do that, especially if they're laughing and playing and obviously not trying to do anything "wrong".
Like table manners stuff, or messing around with the computer...all of mine "played at throwing food or banging spoons etc" or "played at messing with the computer" and even though it was cute, I gave a firm "no" and consequence, because really, I wanted them to learn from the get go how to sit nicely at the table and not form bad habits, and I didn't want to be cooking dinner one night only to find a broken computer from kids messing with it when I wasn't looking, you know.
The upside to being calm but firm for small things when babies are this young, is that they do learn immediately, so you can go back to playing and quit worrying about that. Also, any time you do discourage a wrong behavior at it's beginning, they learn to control themselves for other things when you say so (yes, even at 8 months!), which helps to keep discipline at a minimum going forward, so even if the small thing is no big deal, it's good to address it in the bigger picture.
Other times, I'll ask myself if it's really OK that they do it, and if so, I totally let it go and don't even try to discourage it, or I just play the game with them, so I don't send a mixed message. Playfully doing things for babies, is often their way of asking permission to do it until you let them know if it's allowed or not. Clear no, or clear yes is best for teaching this age, so you have to choose your stance on tons of little things. You say fighting makes her do it more-your consequence may not be clear enough for her-she probably likes having her chin lifted and being acknowledged even if you feel it's fighting.
Call me crazy, but I do believe based on my own experience, that 8 month olds can learn to push buttons (my 3rd daughter AHEM!) and the fact that she is escalating this when you address it would make me know she could understand a clear lesson and could benefit, but many people would wait until she was older to address it.
We just had a realization that my youngest is now 18 months, the exact age my oldest tried her first blood curdling tantrum (and learned it would ever be allowed after a couple of tries), and since my youngest was so much more strong willed so much younger (tried those from 8 months), she has already learned not to try that a long time ago. So whereas we felt she was more difficult and exhausting from an earlier age and we have had to work much harder with her, we now see she's actually ahead. Phew. But still has the fire in her for sure.
For this case, since she's 8 months old, and you will never turn your back on her in the tub, you could just play with her and let her do it since you can stop her from drinking too much, and when she's a bit older, you can be more firm about it if she still does it, or you can be firm and have her quit doing it. Your call.
Personally, I gave my middle bathwater drinker a very firm "NO" as in "consequence warning" the very first time he did it, and I would have followed through if it came to that, because I just didn't want him doing it ever, and he didn't form the habit. But I know he gets super obsessed with doing stuff, especially if he can tell I'm mildly not in favor of it, so I'm more "clear" with him. Whereas his big sister would always lose interest on her own, at my "suggestion" so I didn't need to be so firm.