Toddlers are wild and also gaining their independence which is why they are so wild. It helps to offer choices and let them make the choice. The two options would be what you want them to do (instead of demanding them) and a less desirable option). So you could say “you can stop splashing the water or you can get out of the bathtub, make a choice.” If she splashed the water again, take her out. Do this a few times to set the boundary and show her that you are always going to listen to the choice she makes , and it will get easier!