Horses do not have good depth perception so what is actually an inch deep puddle seems like a deep black hole to a horse. Horses know that if their legs get tangled up and their feet are fouled up they are done for, so that's why horses are so reluctant to go through water, mud, etc. So we need to understand and be sensitve to the horse's nature as a prey animal.
I agree with working with a tarp first. Play approach and retreat. Send her to the tarp and if she stops, even far away from the tarp, she is telling you she has reached a threshold. If you push her past this point things will get really bad. So now ask her to retreat. Come away from the threshold, re-approach, come away, re-approach, etc. THIS IS NOT ABOUT THE TARP OR THE WATER. This is about building her confidence in you and in herself. The worst thing you could do is push her and get frustrated. How can she trust you if you push her toward the thing she is afraid of? Be patient. Understand her fear. Put yourself in her place. So now, when she finally sniffs the tarp take her away from it completely and let her graze for a minute. Go back and repeat.
So say she goes across the tarp confidently and she is no longer fearful of it. You will do the same thing with the creek, just from up in the saddle. Say you are riding along....you come up on a creek....you feel her start to hesitate and slow down, so now you stop her softly and say, "Lets back away from what you are unsure of." Stop and let her settle. Rub her and when she is relaxed ask her forward. If she has trouble back her up again and wait. If she still isn't going get more insistant, not mean, but insistant, and when she tries take all the pressure off. So say she approachs some and then stops. Back her away (retreat) let her settle and ask her forward (approach). A big key here is to give her plenty of time to think. Don't rush her. Again, this is NOT about her crossing the water. It's about her confidence. When she crosses stop, turn and face her to the water. This is helping her flight line become shorter and she can assess what just happened.