It's basically simple...set your dial on M setting & use your TTL (through the lens metering) to adjust the settings to get the right metering...if the tick mark for metering is in the middle fo the scale, then you're lighting will be fine for the exposure you've set.
For water flow in BRIGHT day light, you want a HIGHER fstop, (apature, which is like your pupil, gets smaller with more light) say, 22 (smallest setting on my cam), my Canon has a lil' * for the Apature setting. I then dial it up to get to 22, then I press the shutter button half way to see if my Shutter time (AKA Time Value (TV), like your eye lid, can remain open or closed, or blink real fast) is correctly metered. If my time value is too short, say 1/90th of a sec, the meter will tell me that my exposure will be too dark (+2), & I can dial it down to the more appros TV, you can watch your metering scale drop or increase, with each coupla click either way...
If you need more help, lemme know...
