When I'm adjusting my horses to the trail, I always keep this in my mind: "My horse will do what is the least amount of work."
If my mare balks at the edge of water (or any "scary" obstacle), I turn her around quickly and either lope the other way, or turn her in circles. Then I stop, and ask again. I repeat until she sees that crossing water is MUCH less work than loping and turning circles. :] It usually only takes one session of this to get them used to it, because they A) realize that it's easier to do what you're asking the first time, and B) realize that it doesn't hurt/isn't scary, and will trust you to make the call in the future.
However, my mare REFUSES to cross railroad tracks - she trusts me more to back her up over them than she trusts herself to walk forward over them. So we have to go backward over railroad tracks... 0.o