Sir SR is right, you can use manual with CLS, so the on camera (master) flash sets off the slaves. You can then set power from the camera for the external flashes, if they are compatible.
I think even on full power the duration of the flash will be short enough to "freeze" droplets.
It will be a bit hit and miss without a trigger, but indeed with digital you can afford to take lots of shots.
Don't know if you are into electronics at all, but if you happen to have an Arduino you can program it as a trigger, and have a cheap solution. All you need to add is a lightgate (and of course the work in setting up the Arduino).