when you say you "recently" got this horse, how recently? How much do you think you have worked with this horse up to now? Horses are not scared of water, they drink water they will walk through it on there own at will. (its madding isn't it). I think that you and your horse have work to do it shouldn't matter what is in front of you, water, tarp, jump, stairs, road, bridge, ditch, or cliff. If "your" training is where is should be you ask the horse to move forward if you have done what you need to do, your horse should move forward. Just because she is "great" at everything else doesn't matter. You have found one of the "thing" that you horse feels is too much for you to ask.
As for working with her to cross the creek. If you haven't had her very long I would skip it do lots and lots of ground work, working on obeying and trust. Once you feel you have some very good ground rules and your horse know you are the boss no matter what, then come back to the creek.
If the creek has ANY embankment to it I WOULD NOT back you horse down to cross the creek. (IMO) FOR ME I would back a horse into anything to get them into it BUT that's just my opinion.
When I am getting a horse to cross water or whatever they don't want to walk up to. I point their nose at the thing and ask them to move forward (keep a good seat don't lean forward). So I ask them to move forward, the ONLY time I move them away from "the thing" is when they move forward. So I ask them to move forward, they take a step in the right direction, I release the perish, if the horse is getting too worked up I will ride away from the thing (I might just turn them away from the thing), give them time to rest a few seconds then point them back at the thing, and ask again. So on and so on, over and over until they cross the water, step on the tarp, smell the scary rock...whatever the "thing" is.
But, if this is a big fight I have to decide do I really need to fight this here and now, can I work on this problem in some other place, what is the training that is lacking in my horse that is holding them back from doing this. ITS NOT THE WATER its a training issue. let the water go, think about training.