Hello,
... I want to photograph sunlight reflecting off a small stream, with a few small trees on either side. ..
Thanks!
If you expose for the reading from your TTL meter, you will get dark, oily looking water.
Just imagine, how much brighter do you want the water to be and ADD exposure (open up the lens or extend the exposure time).
I think -to get reasonably "natural" looking water- you would need to INCREASE the exposure (above that suggested by your meter) by about two to two and a half stops: this will give you bright water with visible detail. Give it a go!
Another way of doing it, would be to look for something mid-gray (weathered wood, bitumen, gray stone) in the same light as the water, take a reading on that (get very close and measure at right angles to the sun, avoiding glare) and expose accordingly: the tonality of the water will then fall naturally into place, but (depending on the exposure latitude of your film) the details may be blown out.
With this technique, the ISO of you film is relatiely unimportant, except that -as I understand it- with colour negative film, the faster (ISO 400) films have a greater exposure latitude (can cope with a greater subject brightness range) than the slower (ISO 100 or 200) films.